"Apparently the ship can't turn and can only go forward, with a bit of steering."
You are correct, this is something my heroes often complain about that I explain by saying the map is too small for the ship to turn meaningfully. It's a design flaw that isn't easily fixed without changing the course so it isn't squares.
"The rocks sink the ship oneshot, and are hardly avoidable right at the beginning on the battle map we got (Wild Vortex)"
Your crew has some options here. The obvious, and kind of difficult one, is scoot the ship towards the vortex until it clears that troublesome chunk of three rocks. At that point the ship should be able to avoid the vortex and/or flee.
I made a house rule where players are allowed to roll to stabilize the wheel to prevent the ship from being dragged by the current. So long as the number of movement points rolled by the hero holding the wheel exceeds or is equal to the distance tugged horizontally by the current, the ship doesn't move. This helps avoid unintentional death that does seem pretty unfair.
About fleeing, since the rocks are placed rather inconveniently for a quick escape for the Revenge, I think a loophole in the rules is for the heroes to simply swim off the edge of the ship. While I didn't allow this for some time, in rereading the rulebook, I found something weird on page 18: "a lieutenant may flee off of the board through an edge of the map just like a hero." While the game doesn't really say heroes specifically can do this before this part is mentioned, it does say that lieutenants can leave off the edge of the map, just like a hero, so it follows heroes can leave just like lieutenants. A bit of a lame escape, swimming away from your ship. Surely you wouldn't die that way! But apparently it's allowed, and I don't like ignoring rules that help the heroes.
"The lieutenant (Siren) can move freely 10 spaces in the water and go behind the ship."
While I would prefer hiding behind the rocks to try and draw the Revenge closer to an accidental death, going behind the ship is a pretty clever trick. The quickest solution would be to drop anchor to prevent the Siren from taking too much advantage of a blind spot, or you could always flee. A riskier attack plan would be to try and use the cannon's Knockback ability to maneuver her into the center of the vortex, immediately killing her. Since she can swim though, it's unlikely she'll be dragged in.
"Because of this the players are forced to swim ti go hit the siren, but they don't have Swim and the current is moving them in the opposite direction."
Sounds like the players aren't placing themselves so they current does their work for them, allowing them to move towards the Siren rather than past her. In general, the best solution for this appears to be to flee.
"The cannon is also useless here because of distance and accuracy."
Other than giving the Overlord a pat on the back for avoiding some devastating cannon fire, my only concern for your heroes is the singular. Tell them to get some more cannons. If they can't kill the Siren on this encounter draw, they should flee and draw a new zone.
Overall, the best solution for the heroes here is to run away. The Siren has quite an advantage if the Overlord is willing to wait, but the heroes can be patient and wait her out too with carefully placed rest orders (trust me, my heroes did it to me). Another zone will probably present a more favorable outcome for the heroes (see whichever terrible zone lets them control the sharks). Outfitting the ship properly should happen quickly too to prevent the Overlord from having too much of an advantage at sea.