My Boat motor wont start?

I just bought a boat and it ran fine during the test drive.... now the first time on the water and it wont start. It tries to turn over but has a hard time. When it was in my driveway it had a little problem starting but then fired up several times.... when i got on the water it struggled to turn over and wouldnt start ..... i have 2 batteries 1 of which is new..... i tried to turn it over by hand and could get to turn a little bit but it would get stuck after about a 1/2 turn....... is there something i can use to help me turn it manually and what could the underline problem be??