In my opinion you would want to get a bottle jack that fits under the axle. Keep in mind that the axle will be lower if the tire goes flat. A rolling floor jack like tire shops use would be even better but takes up a lot of room in the tow vehicle. They are better because 1. they fit under an axle with a flat tire on it, and 2. you don't have to reach under the trailer to pump it up therefore a lot less injury exposure if the trailer rolls off the jack. If you raise the frame with a jack you have to lift it extra high to get the tire off the ground as it "hangs" from the suspension. Get one with a lift rating of at least one half the weight of the boat plus trailer. There will still be weight on the tongue as well so one half should be plenty. If you get into a situation where the jack won't fit under the axle (as in a flat tire that lowers the axle) you can pull the trailer onto a board/ramp that raises the axle a little higher. Chock the wheel on the other side of the trailer and set the parking brake on the tow vehicle to help prevent the trailer from rolling off the jack.
We have a three axle trailer with torsion bar suspension so it is super easy to just pull the trailer up on a block with one of the other tires on the same side. The flat tire just hangs in mid air until you drive it back off the block. Its the ONLY good thing I can say about torsion bar suspension.
