Since this is a used trailer and you intend to submerge the axle in water, I would check the tires and wheel bearings to make sure they are in good condition, re-pack the bearings them with waterproof bearing grease and then install "Bearing Buddies"....they need to be filled with water proof bearing grease that is injected under pressure with a grease gun.
If you are handy, this is easy to do, otherwise, impose on a mechanically minded friend or take the trailer to a good tire shop that does wheel bearing replacement....they can check the bearings, replace if needed and install the Bearing Buddies.....you can purchase a grease gun at any auto parts supply store.....just make sure the grease is "waterproof".
For peace of mind, you should also buy a spare tire mounted on a galvanized steel rim.....these are usually less than $50 and mount it on the trailer frame......you can see photo's on the Trailex home page where the spare has been mounted.......a mounting "U-bracket" is under $10.
Last is a lug wrench that will fit the nuts on the trailer wheel assembly.....since the trailer and kayak is so light weight, a jack is not needed to lift the tire off the ground in the event of a flat tire......I carry a block of wood to place under the suspension and hand lift the trailer up by the aft cross bar.....don't lift by the fender as it's plastic and will break.
When you get home or if there is a wash station where you launch, a fresh water wash down of the trailer helps to prevent salt water corrosion.
Good luck and enjoy your new Hobie.
Hood River, OR