1992 SNOB Possible Transmission Oil Leak

Hoping to get some good old fashioned wisdom on PCM transmissions. So bought the boat about two weeks ago and ran it twice without problems. Boat shifted beautifully and never noticed any slipping issues. Last time I ran it I got stranded since it wouldn't start over. Suspecting it has to do with slipping alternator belt since it was squealing. Anyways, back to the transmission. Last time before I put it in the water (foolishly) I checked the transmission oil for the first time. It looked scary low. Seeing that it called for 20 20w oil and I couldn't find it anywhere, and with the help of super nice people on this forum, learned that I can flush it out and put Dexron III in instead. So, I did that and before my battery died it was shifting fine for about 30 minutes from neutral, idle, WOT, reverse, etc. When I brought the boat back to the garage, the next morning I did a cold transmission fluid check, and it was in between the empty and full marks. From what I've been reading it should reach all the way up to the little rivet almost halfway up the dipstick on cold readings correct?

Also worth noting:

Embarrassed to say this but I've never flushed and filled up transmission fluid before. I was very nervous with over-filling it so it's entirely possible I didn't put enough in. Every time I put in more I was getting completely weird readings on the dipstick, which I attribute 100% to user error. I've decided to not run it again and just let a marine mechanic who knows what he's doing to drain it again and fill it up completely and (hopefully) I can watch him so I know I'm doing it correctly.

My question to you guys is how would I check for a transmission oil leak? I've heard of people saying they put cardboard under the tranny and check for leaks, but won't water get on the cardboard? That doesn't really make sense to me. I guess another thought would be to completely clean the bilge and then check the bilge water to see if it looks like diluted transmission oil? And to that I'm hoping someone can explain how they clean their bilge if it's dry and on land? Because right now my bilge has oily residue on it (I'm assuming) from fogging oil and what not. Can I just go spray water around the bilge willy nilly? Or do I have to be careful that no water gets sprayed onto transmission, engine, etc? 

As you can probably tell I'm a super novice when it comes to all this, and just trying to keep the boat healthy without doing ignorant damage to it!

JQ

I'd make this guess that if you had a piece of white cardboard or a piece of white paper taped to the cardboard you'd figure it out pretty quick.

Just a guess 

Run the engine on the hose until it reaches operating temp.  BE CAREFUL if you decide to run the transmission "in gear" on the trailer.  If you choose to do this, you must keep the strut bearing lubricated constantly with a stream of water.  Obviously you need to be cautious with a spinning prop.  Make sure the fluid level is topped-off before running.  And, NO, don't go to WOT.  Don't exceed 3,000 RPMs without a load on the engine.  After you shut down, then slide the cardboard under the tranny to catch any drips at specific locations.

JQ