From doing sea battles you get heroism rep. After this you can purchase items in the shop, the first ones are "horms" when you reach the first title of Vandal. Upon inspection, it would appear that the items are class specific... meaning as a witcher I may only use the witcher horn. However the description says that you can not use if you have zerker or paladin jewellery.
Basically this rules out most people on the game as they choose to balance their class by having accesories from another class. Is there any way that an item could be available that you can use with mixed jewellery? Otherwise I see no use in gaining this particular reputation. The sea battles are fun but it's going to be costly getting the rep up when the only way I'm able to use the items is by changing all my accesories to witcher and I have no intention of doing that.
(I'm not alone in this, you'll find very very few people with just their specific class of jewellery)