Desalinization plants.

I am having it in a 30-foot design right now....

Water makers as they are called in boats have been around for decades in a variety of size from manually operated emergency devices to highly automated systems capable of producing thousands of gallons a day.

Yes, they're call watermakers. What size boat are you talking about putting one in? I like a system large enough to fill the water tank in 4-6 hours. If you have enough electricity to run it and space to mount it in,

We have 2) 300 GPD systems on an old Bertram MY. RO membranes do all the work and our CAT high pressure pumps are remotely mounted. We only do one or two open water cruises a year and it's great not to worry about fresh potable water.
Our units are of old design and the backup attitude has paid for itself. A newer single unit should probably out perform our stuff and prove more reliable.
Look back and search RO systems on YF. Some (current) models do have issues (LCD displays). If I remember, Pascal & Skippy J may be of help here.

Have you checked into tieing both membranes together? You might be able to run one system and tie together both systems membranes with a valve and get a lot more volume. Usually the only difference in some systems is an extra membrane.

It's not been a pump or membrane issue. We have an old HRO and older Rich Systems. As simple as there logic designs are, the boards have had problems.
BTW, the Rich brothers still did answer e-mails up a couple of years ago. Their original manual and their support was fantastic. I finally figured out the schematics provided and can gate chase and repair on my own now.
The HRO unit has some weird chips and as much as I have tried, do not cross to available gates or regulator chips. Whole boards are available. I'll figure them out one day. BTW, Factory support sux from HRO, the kids at Beard (in Liquordale) have provided parts and some good support. I do like the product though it's 20 years old.
The Rich system near 30 years old.

I was given an old manual 12Vdc pump and mini membrane system. Since were usually on inverter (no gen-set) while moving, I thought of attaching it to the ship somehow and make water as we motor (offshore) around. Only 5 - 7 gph but after a long run, lots of water and micro hp consumption.

Back to the O P; RO systems are here, compact, some affordable to us rednecks and not really high tech.
If you're on a military sub, it is a big deal and would require a mate to monitor it. Lives (missions) could depend on it.

The nuclear submarines I sailed in in the US Navy used the distillation process, as opposed to reverse osmosis (RO), to make fresh water. They had a 6000 gallon-per-day (gpd) still for the humans and a 2000 gpd still that made purer water for the power plant. That might be the reason for the size that you noted in the documentary you saw; comparable RO units would probably be smaller. Although, just about everything built for submarines will be overbuilt compared to their civilian counterparts -- submarine stuff is built to be sailor-proof.

Does anyone have info on small distillation equipment? I think many years ago some yachts used distillation, before the current method with filters existed.

It seems the waste heat in the exhaust could be used for distillation.

Google Marine Evaporators, These things are designed to be used with a constant source of heat and electricity.

For most of the vessels folks on here have boiling a pot on the stove and collecting the steam to condense it might be the cheapest way to get some fresh water out of sea water.