Finding a leak, have started basic trouble shooting

I have a 96 Marada 21' open bow I/O with 5.0 V8. I recently got a leak that I can't find the source of but know its taking on more water than I'm comfortable with. I have the boat on a trailer now and yesterday I tried putting the plug in, have the front of the boat raised, and used a hose to fill the bilge area with water and there was no leak. I drained that and put the boat on muffs, put the plug back in, and ran it for about 10 or 15 minutes with no evidence of the leak. I have divider walls on either side of the engine for storage so it isn't impossible but is difficult to remove and inspect easily. My next thought was to see if I have easy access to the back side of the trailer hook on the front and put baby powder on it to see if maybe I have the leak there next time I'm in the water but for confirming if its the bellows or not that seems like a difficult place to view and confirm. I bought the boat last september and other than some vinyl damage the boat seems decent overall. Is there a good place to start checking for the leak with the boat out of the water? or is it worthwhile to do something that requires the boat to be submerged? (I have been searching other threads on the topic, just want some additional thoughts based on what I've already done)
Tahnks

Re: Finding a leak, have started basic trouble shooting

I'd be looking at the bellows if it is only happening when in the water.

Re: Finding a leak, have started basic trouble shooting

The cooling system or perhaps your rub rail seam all around the boat would be good suspect areas to inspect. Verify that all your engine hoses are well connected, them inspect your bellows by raising/lowering your out drive, turning the steering wheel may help you find the leak using a flashlight while it's dark. Have you replaced the bellows? any maintenance records? I would be inspecting the bellows each year to ensure they are in good condition regardless, these are wear items that will need to be checked each year.

Re: Finding a leak, have started basic trouble shooting

Also, inspect all thru-hull fittings for leaks around the fitting or at hose connections. Check front of keel for signs of excessive beaching which can create punctures. I agree with the potential for the leak to occur at the rub rail. Do you have any punctures to horizontal surfaces ie spider cracks, missing snap fittings, missing cup holders, holes from older electronic gear, bad gaskets on access compartments? Good Luck

Re: Finding a leak, have started basic trouble shooting

the rub rail seems like a good idea. I'll just do some preventative maintenance regardless on that. I'll try moving the steering back and forth. Maybe I'll just take out the divider walls and get down and dirty with the search. thanks for the advise so far. The hoses I can see easily seem to be in good order and a mechanic friend advised the same thing recently. but I don't have maintenance records. I may just have to suck it up and pull the out drive and check those connectors.