Access to the area under the shower requires extreme agility. You can also remove the airconditioner to get slightly better access. The floorboard just in front of the housing of the A/C gives you limited access but you will need to execute a Lambada dance for it. You said it’s fresh water, my guess is the condensation drain for the A/C is playing havoc. You can remove the wooden cover of the A/C pretty easy, 4 screws. If your shower is leaking the smell will tell you ...
This is also the place where salt water will go if your rudder gland is leaking.