Motor Running at low RPMS

2000 Honda BF8A
A little history
A few weeks ago I took my boat up to moss lake, where my aunt lives. I fished one weekend, pulled the boat out of the water, and threw the cover on it. I left it at my aunt's house for the week because I would be fishing again the next weekend. Next saturday morning, my uncle and I went down to go fishing. After we got everything together, we were about to launch when I noticed about 5 gallons of water (it was still dark) in the back of the boat. Anyway, it wouldnt plane out 1st thing that morning, I figured it just needed to warm up. Soon enough, it warmed up and got up on a plane. Ran fine for most of that day. That evening we went back out, and it wouldnt plane out. I know its low rpms because, when its in neutral it runs fine, but when i kick it into gear it slows down. I figured it was just water in the tank, so I drained it,let it dry out and out some fresh gas in it. I am still having the same problems. Just to give you an idea of how bad it is; running normal I get about 19mph on the GPS, now im only getting 5.4mph on the GPS. I plan on taking the motor into the shop, but tell me what you guys think the problem is. Thanks

did you check your spark plugs to see if they look good? maybe that'll help hopefully.

Mine did the same Saturday with the exception I could go full throttle and hear the RPM'S go up just the boat would stay at 4-5 MPH like yours. Wish I could help.

I just had another thought, because i'm also having the same problem (made a thread too). Check your fuel tank for air leaks.