h2o tank leak
I am assuming that this is your only water tank? If so, unless you really want to get your water from jugs, then I'd guess you don't have much choice.
Pull it apart & see what's going on. Most likely the caulking around the outlet has deteriorated; clean & replace with fresh caulking, NOT 5200 (it is a glue, not caulking). Sometimes the plastic fittings (normally thru hull fittings) crack & leak, too.
But unless you have deep pockets, you are going to have to tackle jobs like this, as they come up, yourself, if you want to have a boat.
Good luck & let us know what you found.