You forget a few vital differences here:
1. On land you can pimp your horses, get five speeders and then outrun all mobs, you cannot pimp your ship or hitch anything to it to make it go faster. You are totally reliant on the wind. And in wurm we have the light breeze, and the gale in your face.
2. On land you have roads and guard towers built, in the ocean you have to rely in the occasional guard tower which happens to be close to water in most cases they are inland. Due to the slow speed of guards you have to sail around the tower for quite a while before they reach the serpent, so you have to navigate to go in circles there with the wind which affects your speed thus you may be sailing right to the serpent instead of away from it...
3. You can have the wind in your face too if you try to get away from it, slowing you to below 5 km/h. You cannot just manoevre around as you could do on land, as you are relying on the wind.
4. The NEVER lose aggro. I don't know of any land mob that acts this way. Most times if you get far enough from it they stop chasing you. Sea serpents never stop chasing you unless you put guards on them to draw aggro from you, or you switch servers. Now, if you happen to get the serpent on say the east side of Release or Pristine, where there are currently three or more sea serpents living, you cannot just sail east and off the server, so bless your soul if the wind happens to come from the land's direction and you try to land your ship to escape the constant attack.
5. They also attack ships now.
6. Two people on a cart can fight a mob together. Two (or more) people on a ship cannot fight together, as the passengers are too far to hit due to their position on the ship. As far as I know, nobody can really solo kill a serpent, unless you are a priest with some spells, and even then, it is doubtful you will kill it alone.
If you want to make these mobs have such huge aggro, make them slow so you can outrun them. Secondly, allow for imping ships so they go faster. Thirdly, get rid of the light breeze. Fourth, make them lose aggro. Fifth, everyone on a ship should be able to hit them even if they are passengers or not. Once these changes have been made, you can start comparing these sea monsters with land monsters, and your options to deal with them.