Small world! We used to keep a boat at Fleetwood!
If you're going to be driving on beaches, there will be salt involved. You don't say how much you want to spend, but you can either go down the "disposable" route and buy cheap, then just throw away when it rusts (or fails catastrophically in some other way), or you can try to spend more and hope that the corrosion protection will do its thing. I hear good thing about Toyota pickups when it comes to durability. The Nissan ones (as has been said) snap in half and rust quite alarmingly. Also their diesel engines aren't the best. Timing chains seem to be fragile. I've never had a Jimny, but I've seen a couple just "skate" over sand where heavier 4x4s have simply dug themselves down to the diffs and got stuck. If you're only pulling a jet ski, the Jimny might not be a bad option (and a separate chassis is easier to weld)! As has been said, wider tyres to "float" the car on the sand are the way to go. Lots of American 4x4s are useless in British mud, with their ridiculously wide "macho" tyres, but mud isn't so much a thing where a lot of these are used, it's sand. If you have a lot of £££ (both to buy it AND to feed it!) the Landcruisers are supposed to be indestructible too.
Down at the "disposable" end of the market, Freelanders get a bad press, but I've had a Freelander 1 for a few years (get the one with the BMW TD4 engine, rather than the Rover engined ones), and it has been better than expected. Sure, a lot of them have been badly neglected, and will be troublesome, but provided the viscous coupling hasn't seized and the propshaft hasn't been removed (which sellers will tell you is for "fuel economy" but is usually because the coupling has seized and trashed the transfer box or the rear diff), they're quite capable off-road. However, they lack the low ratio box of a "proper" 4x4.
Alternatively, just use an ordinary car and launch off the slipway at Knott End or Stannah?