Mariners are terrible, evidenced by the fact that after all these years AFAIK my ranger Trevant (who was a hunter), is still the most "successful" Pk-wise mariner to date.

Mariners, IMHO, would be fine under two circumstances :

1) Netcasting works better.
2) That lightning spell they have actually works/does something. That was totally broken when I had it (broken as in you cast it and nothing happens at all).

They don't need anything super epic, just fixing those two skills would make them decent. Netcasting, a whole lot of times, just did nothing noticeable at all. I think its too easy to save against or whatever. Stormcalling never worked when I had it, if it actually works now and does call lightning multiple times on people then that's cool.