When I played a mariner, I thought the neatest thing about it was being able to camo on water. That's huge considering that most everyone has to cross the continents at one point or another. You're only giving up snare, charge, and pathfinding, all of which are given equivalent replacements. Snare - netcasting, charge - harpoon, pathfinding - navigation.

Suggestions for improvement, if anything, could be giving them the drown spell, letting entangle work to lesser effect on land, allowing the ability to scan on the Open Sea, and maybe strengthening stormcalling. Otherwise, you're able to camo and creep in the one place that every character has to pass. Outlanders and their enemies cross regularly, there's a ranking spot or two you have to cross the water for, and, to get to Seantryn, Veran, Basilica, Trothan, Vaults, etc., you're going to have to cross the water.