We have found this really useful for single boats - make sure that you buy the 'Large' model as it provides extra ground clearance for your fin.
The axles will wear out over time but ours has lasted 2 years so far and a repair should be possible.
Take care of the wheel pins - replacements can be found online. Stainless Steel '316' marine grade are best.
Use a cheap roof rack strap to hold your boat as you move over the beach bumps.
Search around for a good price.