Newbie needing help - no power

[MuleTrain98]

Hello, I have a 2001 GTI. When I put it in the water for the first time this year, it would only go a little faster than an idle pace when I held the throttle all the way down. I ran it for at least 15 minutes, hoping that maybe the engine was just cold and needed to warm up some before it would kick in, but it never did. Any ideas on what could be causing this?

Thanks,
Brad