Ferro-cement potable water tank repair

I am helping an elderly couple research the problem and had to rendezvous with them for pictures and more info. Picture follows. This is a synopsis of the situation:

The tank is under their house, part of their foundation.  It sits on the beach, in the salt water at mean high tide (in contact on average up to the 3’ mark when the tide is in), it is ferro cement water tank, was supposed to have lasted 25 years, but is now approximately 40 years old. It was custom built in Tacoma, WA. It is 10x10x9', has 1 baffle at the 6' mark, approximately 10,000 gallons. Construction consisted of 1/2" rebar on 6" centers, expanded metal lathe and the addition of a lot of anti-hydro.

The damage is approximately 4 years old. The owners are 80 years old and want to repair rather than replace the water tank. He is very handy and is prepared to do the work by himself. Unless we learn differently, he is prepared to empty that side of the tank, grind the inside area, apply the only product he is aware of "Splash Zone", and then paint the area with a 2 part epoxy made for water tanks.

Any advice you might have would be greatly appreciated!!

i used drylock fastplug  on areas in my basement, i think it should work for this also.

most DIY stores would have it

Not bad for 40 years , especially exposed salt water. If the issue is water leaking out, which it looks like to me, and that leakage is minimal and the owner is 80 I would leave it. Getting in there and grinding will contaminate the tank and an epoxy  will not likely stick anyway. You will never get it dry enough with the tidewater at it. It would take a week or more to "maybe " dry it out enough for epoxy and minimum epoxy sets usually run up to $200.00
Or as Phil suggests , plug it from the outside.  By the look of the efflourensence  the water is not exactly pouring out and there is probably no shortage of water either in the area ?
Thats my assessment, without knowing more.
I am a mason by trade .
Mark

I would avoid using any latex or water based "water proofer". My suggestion is to read the label carefully of whatever they consider possibly using. There is a big difference between damp proofing and water proofing, and some of them are only intended for the positive side (the side where the water is entering the crack) or the negative side (where the water is exiting the crack) and some are meant for above ground use and some are made for below ground use. Some can truly waterproof and hold back hydrostatic pressure, a lot cannot. Some are very harsh and nasty solvent based asphalt toxic gick, and some are a lot less offensive, to both people and the environment. There is a product called Tamoseal and it's a special cement and comes in a sack like regular cement/mortar/concrete, and it must be mixed with a special acrylic admix, but it trowels on just like a portland cement, and has a zero permeability rate, and can be used below grade. Whatever they choose, I would abrade the surface so a quality bond is made so the repair works and lasts. Hope this helps!