I too, have a 1990 Fish and Ski which has a leak I have yet to identify. I also have to bilge frequently to keep the water out.
So far, I have plugged both live wells and filled them to confirm no leaks from the livewells themselves.
Haven't gotten around to checking the plumbing, but thinking if you open the main bilge port (in the center) and then plug the others (2 intake and 2 drain for each of 2 live wells) and then fill both live wells with water would be a good way to test the plumbing for the live wells. If the problem lies there would be water coming out the main bilge...
Bought it at the end of the season so haven't had a chance to try this but just passing on my thoughts. Is currently in storage but when I get it out for the season and give this a shot I will update this post.
Another thought is getting plugs that will fit all the live well ports, then take it out and see if any water is bilging, if no water bilging then leak IS likely with the livewell plumbing since sealing those drains would effectively fix the leakly live well plumbing.
Outside of that my next likely candidate is the ladder, I am betting that is the source but haven't had a chance to test it.
Anyway, just another fish and ski owner with a similar issue, will keep you posted what I find when I get it out for the season.
Also, here is a link to a thread I had to figure out which ports are which on the back in case it helps...
https://www.nitroowners.com/threads...hull-ports-90-nitro-fish-and-ski-110hp.84035/
Thanks,
Geno
PS just noticed this post is from 2009, so likely not still active, but please feel free to post here for thoughts on this issue...