1978 V20 Steplift trying to find leak?

[BeachBum]

I would like to start out by saying I am so thankful to have found and became a member of this website.

I just purchased a 1978 Wellcraft V20 steplift with an outboard 200hp Mercury Mariner hanging off the back. Just trailered her from Hinesville GA to her permanent home in St Augustine FL last weekend. Needless to say that was a journey both fun and scary. This is my first big boat and she needed some TLC to get moving on the trailer ie... new tires, bearing and buddies ect......

My brother and me just took her out Sunday on the ICW and she roared. just a few stall outs in idle as many people with mariners have said thats routine for this motor.

Now to my problem: When we started the day out in the AM the bow was sitting up high, nice and pretty taking on no water at the transom not even at put in. As the day progressed the bow got lower and lower and preformance was hinderd from around 40mph to about 32mph. At pull out time once the boat was on the ramp and out of the water upon pulling the drain plug atleast 8-10 gallons of water drained out. My question is to anyone that has had a similar problem could throw some advice my way on how to start the leak search process. I also want to ask about some fuel tank issues as we had to run with an external fuel supply tank being that the onboard tank is unhooked. Do you think this is because of a leak in the tank? If yes then how would one safely go about confirming that?

Thanks In Advance
BeachBum