The reason we're tackling this one now and not earlier is because although you can see it from the first island, you'll get a 6-star wanted level if you attempt it before opening up Alonquin. Now, the best thing to do is line up your bike perfectly before you tackle the jump - then just back up without touching the steering at all. Now, assuming nobody gets in your way, you'll be able to take the jump knowing you're going to land in the centre of the bridge - the only place it'll register as successful.
Nothing tricky here, although there are loads of ramps in the water, so you might have to try a few before you find exactly which ones trigger the stunts. Check the video if you get stuck.
We couldn't figure out exactly where we were supposed to land on this one(which is up at the north of the island near jumps 17 and 18), as every time we landed up on the road it said 'not good enough'. So our advice is to follow the run-up in our video and just go as far as you can. It triggered for us in the water, so that's good enough.
Check out the video for all the run-ups.