Re: changing tire size
I'll answer your question since the others didn't...
Your first issue will be fender interference. Make sure the fenders will not come into contact with the tire at full suspension compression.
Your next issue will be bolt pattern and size. Most, but certainly not all, trailer wheels are 5 (lugs) on 4.5" (bolt circle). With the smaller wheels you sometimes find only 4-lugs. You will want to make suire your 14" wheel fits your hubs correctly. This may include that your hub is of the correct diameter. If you have a 5 on 4.5" hub already, I don't suspect you will have any issues. --IF-- you need to change hubs, make sure you get the right bearings for your axle spindle and new hub.
If load capacity is your only issue and not ground clearance, then follow SOTW's tire suggestion in message #5 above.
Lastly, if you go with a larger tire with a higher load range, you will need to experiment with your tire pressure a little bit. Too low of a pressure and your sidewalls may overheat and blow your tire. Too high of a tire pressure and you will beat your boat to death!
Let me know if you have any more questions!
Dino - KX6D