Roger-
Sounds "Ohminous" !
Here are the things that I know can be done:
Raising the rollers may be a distinct possibility. Check the supports under the rollers to see how much adjustment is left on the sliding arrangements with slots. You will have to have room to raise them all somewhat uniformly while elevatiang the rear. You could also get new supports with a longer or greater adjustment range.
Use bigger (taller tires) for a small gain, will also help carry more weight.
Get larger rims, from 14 to 15 to 16.5 rims- bolt pattern has to match, but for the larger sizes tire/wheel shops can come up with them. This will be your easiest and cheapest alternative, except for the rollers.
You can change axles, but they won't raise your ride height in themselves, though, since they're going to be on the same centered spot under the frame. However, a different axle could give you a different bolt pattern for the wheels in a larger size, though.
You could block the frame up above the axels further with some sort of block kit used to jack up (lift) pickups, but the trailer boys probably would't recommend it. Might be hard to keep the axle(s) in alignment with the trailer frame (?)
If you have a single axle, going tandem would help you carry the wieght, however, and taller wheels could give you the height. If you're adding a lot of weight on a single, a tandem (second) axle and larger wheels would be the way to go. Dual axles would also help bottoming out problems, which are the ultimate clearance issues.
You could also add a leaf or two to the trailer to stiffen things up and raise the clearance. Adding shocks could also help to raise bottoming out clearance, depending on how they act (double or single). Air assisted shocks could also be used to increase clearance and handle more load.
Perhaps a suspension shop could add air bags between your axles and frame to raise the ride height and stiffen up the ride.
If you have fixed tangs, trailer shop could also increase the size of the traingular tangs that the ends of the leaf springs ride in, if major surgery is advised. Many trailers, however, having sliding U-bolt arrangements to connect the leaf springs with the frames to allow fore-aft adjustments.
Lots of stuff could be done, depending on the creativity and knowledge of the person helping you.
You may have to shift the axle(s) back on the trailer in any event to keep the boat balanced on the trailer and the tongue weight corrected.
If you have torsion axles or coil springs, some of these things will work, some will not.
Back to "Square 1" and sort out!
Sounds like fun!
Hope this helps, and is all the the best of my knowledge,
Joe.