Chiller recommendations

Very odd that you would lose both at the same time. You've had a few maintenance and equipment age blips in the past with steering, Gen. alternators not holding load , controls and water pumps but certainly nothing as financially painful as this will be IF you indeed need to replace both chillers.
Depending on the manufacturer you can expect to spend 25,000 to 35,000 + per unit and the un-install / new install will be a crap shoot numbers wise for labor with the foot print size and plumbing connection matchups , fabrication of any needed brackets etc. so I would estimate 2 techs @ 4-5 days labor at an absolute minimum of $95 per hour so figure $6 to $8,000 for installation minimum. This is going to painful $$$ and seeing that the vessel is for sale I would do everything in my power to save what you have if at all possible.

If the chillers have to be taken out and replaced than you could certainly save some money by going with a custom boutique manufacturer such as Flagship or Ocean Breeze (both located in Stuart, FL. or go with a well known manufacturer like JD Nall Dometic etc. but you'll not get a 5 ton or 65,000 BTU unit for $ 15,000 from either . I purchased twin 80,000 BTU chillers in a vertical stack frame from Ocean Breeze last yr for $28,000 and the unit has performed well so far. Of course that $$ was for the units and control box only, Pumps VFD's and plumbing were extra.

I would put new Dometic's in. I don't mess around with any off brand units. That being said your failure is extremely strange. But, I've seen many people over the years go the cheap route and bandaid and bandaid and spend good money after bad, constantly repairing units that should've been replaced 2-5 years earlier. Installation cost is just going to depend on what exactly needs to be done in order to install them.

If your inner tubing that holds the Freon perforated like ours did it is not repairable, dumps the Freon out with the raw water. If the tubing coils are good you could get them repaired for a lot less than new, worth checking out. If perforated the likely cause is too corrosive solution used to clean the raw water side. If your system is still supported by the mfg and makes good equipment it will probably cost less to just replace or repair the chiller units rather than a different mfg, all new controls, and replumbing etc. Not sure I would do major repair on an old chiller unit. Our Aqua-Air system is still being mfg'd after 20 years, so not a problem with replacement and repair.

If you fix junk, you will still have junk. If they're 10 years or older, replace, because you will spend 50% of the cost of a new one and it will fail again.

Aqua Air has been around for 50 yrs plus as this company was one of the pioneers for marine A/C started by James D Nall in the late 60's early 70's and was standard equipment on most new builds around the world including Feadship builds of the '70, '80, and very early 90s. JD NAll was taken over by James son Dwayne Nall and morphed into Aqua air . Decent equipment but has been surpassed by the big players such as Dometic and H&H etc. Aqua Air still takes to many shortcuts like using Capillary tubes instead of expansion valves. Rudimentary control systems with inexpensive sensors that are prone to fail. There is value to be had by a small custom builder such as Flagship and Ocean Breeze as they're local and have excellent tech support and parts off of the shelf for same day shipping if needed.

I feel for you bro!! If you're replacing the chiller, also check the sub-systems; AHU's, chill water pump, salt water pump, and the plumbing for the chill water. With my recent education, I discovered that my 7-8 year old system was improperly installed: The chill-water lines are too narrow, almost 50% of the T-connections were leaking, and there were leaks at the AHU's. The CW pump was trashed/leaking, and the SW pump had too large of an impeller.

I went with Dometic, but there is still a delay in one component.

Aqua-Air gave us prompt technical service which help us thru several different problems we encountered. They also had equipment in stock with fast shipment. Their equipment is good quality. All of the various problems we encountered were not their fault, it was from improper/lack of maintenance and I had never seen a chiller system. Because of the way our problems played out we stayed with Aqua-Air, one thing led to another and we will have almost all new equipment except the loop and thermostat controls. Also had a rupture expansion tank, which was full of crud and some nasty looking water, this was feeding into the loop and causing some of the discoloration, plus not maintaining pressure properly. After your post I looked at Flagship chillers, mainly because Pascal installed their system in his boat and has high regard for their chiller system. Their system looks to be high quality and simple which is what I prefer. A lot of this stuff has gone too high tech, narrows the field of qualified techs.
Of course we hear a lot about Dometic, might have used a nearby dealer but he does not give estimates, but would start a work order to determine parts and equipment needed plus labor per hour, "carte blanche".

That 's it, standard off the shelf components. When you hear proprietary, get ready!

This. Check your piping size, but on the size of your yacht 5 tons a chiller sounds pretty light.

Adding a ton (12,000 btu) you do have to consider refrigerant line size, electrical wire size (1 ton is ~1HP) plus that ton needs 400 cfm of additional air flow.