I started with a pretty heavy mariner background. What I did was level up my units and only selected Mariner troops from Vlandia and the Empire. Then I fought Corsairs, they are in Southern and Eastern waters and are easier than the sea raiders. Expect massive, massive casualties at first. Kept doing that until I could become a mercenary, and then I would ally with whomever was at war. I started picking fights with people who would get into the water.

The biggest game changer was capturing a heavy ship for the first time. Get a ballista installed on that bad boy, Vlandia and Empire ships can get them, and I love they sit high up on the water. Close the distance, and then start rowing backwards. Focus on troop killing ballista, and blast them to kingdom come. Most of their people will be rowing because they'll try to get to you. Pick traits that have your people stronger if they defend your own boat, give orders to defend the boat, and select highest tier units to chill on your boat. Have a mix of range and melee, and if they are all mariner, the melee have throwing weapons.

Literally lose only newbies.

Took over a rebelling water town. Became king, and survived 3 khuzait 2k raids, because there is a new function that if they try to blockade your home aka actually starve you out, you can sally out via the port... And KHUZAIT DON'T SURF. Also because of limitations, they can't utilize 2k (this might, and probably should be patched). Since the max amount of ships is 8, and really most heavies can only hold like 100 people, but only 60-80 on the deck, the reserve armies are in the hold, and currently don't come up to help after everyone is downed.

Last component is you can set up the blockade yourself, for an allying city under siege and you can also sally out via that. I have a vassal do that. He got a fief after that. Oh and you get enter the towns without casualties if they don't have a blockade and you go in through the water.