I don't think there is a submod for this unfortunately.
Did you buy a ship from the shipwright or do you rent one (just walking straight onto the water)? Sailing in a bought ship loses less morale than renting.
Landing occasionally, even for just 1 or 2 seconds, replenishes your morale a lot. You should get back about half or more of what you lost since you last landed. I think even pirates hate being away from land for too long.
Disable the penalty for not resting in the camp mod options menu to reduce overall morale loss (note that it will freeze at it's current penalty).
If this isn't enough then maybe tale a look at the module system and see if there's a way to eliminate it.
http://forums.taleworlds.com/index.php/topic,251745.0.html. Those .py files are a lot easier to interpret than the .txt files you see in the module folder.
Good luck.
Edit: Okay I decided to try and do it for you since I too enjoy the life of a dena/frankish raider! Replace your simple_triggers.txt with this one and there should hopefully be no more morale loss. It will still give a message about your troops being uneasy, and you will still gain morale when you land (I don't know how to remove this).
https://www.sendspace.com/file/qdua5k
I'm not a modder so this might not work properly. Have fun and split some merchant heads for me!