I went through the same search you did. Some things to consider is backspacing and wheel width. Unless you go with a wheel made for a trailer ie. factory 18"s like whats pictured above you will have a hard time getting just any 18" wheel to fit. Most wheels over 17" are 8"-8.5" wide so If you do decide to go with bigger wheels you may also have to get wider fenders to stuff them under. The other thing to consider is tires. Most tires that will fit on an 18" wheel will not have a sufficient load rating for both the trailer and boat weight.
Both, wheels and tires to fit your set-up are out there but finding the right combo that will fit with out modification to the fenders or hubs is not easy.
Also no matter how good a driver you are you will eventually get in a spot where you will curb rash your wheels, no way around it.
Hate to be so negative but thats what I've found in my search.
I ended up going with 14" wheels and couldn't be happier. I didn't have to buy new tires and the wheels bolted right up. I did have to buy tuner lug nuts to fit the wheel but that was it.
Here is where I got my wheels. I payed just under 5 bills and got them about 3 days after I ordered them. Good quality and great customer service.