Detroit Diesel 12v 92s - Help Please!

Are you talking about the recovery line? There should be a container on that with a cold/hot full line. If that's in place you've got another cause for your overheat, possibly a bad waterpump or impeller. If the watercap was defective you'd have water or steam escaping there.

Capt. J, did you catch that he's adding straight water every day, and that the "Water is clear close to pink".

You're supposed to be adding a 50/50 mix of water (preferably distilled) and anti-freeze. The water is just bubbling off. Drain and refill your anti-freeze with the proper mixture and you'll probably be fine.

I'd recommend adding the recovery bottles. Think about what happens to that anti-freeze that collects in your bilge. Fish don't really get along with anti-freeze.

Anti-freeze is always a 50/50 mix unless it's pre-mix. Straight anti-freeze is too caustic and straight water can't handle the temps. I'd do this asap.

Two things we need to check here;
Day Tank?? Expansion tank?? Are you convinced were talking about the same thing. Can't believe a big expensive 12v92 don't have a recovery bottle.
Bertram water alarm or DDEC alarm? There is a problem if either alarm to going off.
50/50 coolant mix in your car; Yes,, Boat; no.
Unless your boat engines are subject to freezing, 20 to 30% is good. On a boat engine your looking mostly for anti-rust, gasket & seal protection and what little over boil protection it offers.

Properly working expansion tank caps have already closed them selves off before operation temperature. OP states it leaked out at temp & speed.
Lets get this over (under) pressure problem figured out.
If this is a DDEC alarm, what is the error or error code and when does it go off, how is it cleared?
If this is the Bertram alarm, what light is going off and when? How/when does it clear?
If this is the expansion tank, with only one cap (similar to a radiator cap on your car) It could be as simple as a bad cap (or two). I assume 12/92 DDC caps are around 8 to 12 psi. Look for bad hoses & clamps.
I would suggest ordering a set of proper caps, obtaining a pressure testing system and a couple of recovery bottles (if you really don't have them).
If your not conformable working on this, this far, Lets find you a good shop & mechanic. 12v92s are not forgiving and not as simple as your fathers old Ford.

Lets also find a DDC shop that can sell you coolant test strips. You dip these strip into the coolant water and per color and match on a guide, it will report the condition and mixture strength of your engines coolant.
Quicker and cheaper than dumping the whole 20 gallons plus of coolant and recharging.
The strips may also report if block or cylinder gasses have been introduced into the cooling system.

"Day Tank" kept gnawing at me, but I didn't want to go off on a tangent. That 20%-30% really caught me. First time I've ever heard that. Maybe because I run in the NE where we do have freezing concerns, but could you expand on the reasoning for this? However, if he's replacing the mix with a gallon per day of fresh water I have to assume he's pretty low on coolant.
"engine water alarm" was another tangent I avoided, but that's a temperature alarm. If his gauges are working properly I DK why they're not showing high temp if the alarm is sounding.

It sounds like a few people need to read the DD literature on coolants, and also read the specs on coolant chemistry and properties at various dilutions. Both ethylene glycol and propylene glycol coolants are approved. DD publishes a very good guide to coolant specs for the entire range of their engines.

FWIW, the pH of coolant can be anywhere from 8 to 11. Put in terms that mean something to most people, its "caustic" range is somewhere between seawater and soap. Propylene glycol based coolants are not harmful to fish, they biodegrade rapidly and do not accumulate in the environment.

Most coolants are formulated so that a 50/50 mixture provides the best performance comprommise among corrosion protection, anti-boil, and additive "drop out."

RTFM is the appropriate first course of action before following any advice you get on the internet - except this advice of course ...

I've never heard of a coolant "level" alarm on boats like this, only temp alarms. That's why I would question the gauge accuracy. As you lose coolant the temp is going to rise. Your temp may not be going over the top, but I suspect it's rising enough to trigger the alarm.
The recovery bottle will close your system, keeping the coolant from going into the bilge and overboard. and give you an easy way to check your coolant level (you don't want to open the cap on a hot motor),
Anti-freeze is also known as coolant, because it serves both purposes. So whether in the GOM or the Arctic you need it.

C'mon Ed, DDEC controls provide a coolant level alarm. Just because you haven't heard of something is not a good reason to post bogus advice online.

If you are not familiar with the topic, keep out of it for the benefit of those who might think you know something and foolishly follow bad advice.

To the OP: Contact your local DD dealer or authorized service provider. The money you save may be substantial.

Ethylene glycol seems to be not poisonous. Some years ago we had a big scandal in Europe, because some Italian companies made sparkling whine out of ethylene glycol (Anti freeze) and sold it all over Europe as quality champagner. It was called the Asti Spumanti scandal. At least nobody died of it, as far as I know .

Sorry Marmot, but the 1292's I've run from that era (1990) did not have level alarms. When are you going to stop sending people down the CFR rabbit holes to nowhere and trying to drum up business for professionals who may or may not be very professional, and start actually giving people the benefit of your knowledge. I'm sure the OP knows how to call a mechanic. He's here to get as much advice as possible before and possibly instead of laying out $150 an hour for what could be a simple fix. What exactly do you see as the purpose of YF, just to announce new launches of cruise ships and mega-yachts? The OP is asking the advice of the membership because he feels collectively we have more experience than he does, not that we can fix his problem from our keyboards, but that our experience may give him some direction. Do you think that exchanging a gallon of coolant per day with fresh water is a good thing? Why not tell him what your experience has taught you about that, instead of telling him what he already knows how to do (look in the manuals and call the manufacturer)? Try to be productive instead of arbitrary for a change.