Rebman morality is different than that of Amber, but is still fairly well bounded by societal tradition. Here are some highlights, especially where they differ from "standard" surface-dweller mentalities.
Clothing
Rebmans do tend to be somewhat less concerned with modesty than surface dwellers; women and men alike most often go topless, and on the bottom wear only trunks or a simple wrap. Short capes and jewelry are common ornamentation, as are harnesses of waterproof leather to carry weapons or as simple accessories.
When Rebmans do dress fully, it is commonly for one of two purposes. Events of extreme formality, or as courtship play. Some commentators on Rebman culture point out the links between the two.
When above the surface, Rebmans adopt local dress out of consideration, although they frequently alter it to suit their tastes. Common trends in surface clothing are snug-fitting clothing around the torso, to emulate the pressure of the sea, while the limbs are either bared or clothed in loose cloth to remind the wearer of the resistance of water to movement.
Marriage
The nobility of Rebma is small in population, and marriage is much less rigid in caste than many other kingdoms. Marriages between Greater and Lesser houses are common, and Rebmans without House (who are nevertheless considered nobility) can and do marry into the peerage. Marriages between social inequals can result in the newly married couple ending at either level of status, depending on the circumstance of the marriage and the opinion of the families.
Interracial marriages are slightly less common, although selkies and sirens do find their way into noble families on occasion. It takes a significant amount of political and social influence to do this successfully, and Rebman law specifically bars nonhumans from inheriting leadership of a noble house.
Also common in Rebma is the practice of taking spouses from the surface world, either from Amber or from nations of the Golden Circle who sail the seas above Rebma. Despite folksongs to the contrary, and constant rumor outside of Rebma, these marriages are almost always voluntary. The practice originated with the selkies, but has long since spread to the nobility as a way of bringing new blood and influence into the noble families. It is worth noting that sirens also take mates from the surface, but as they have no means of allowing their lovers to breathe underwater, these stories almost always end badly.
Sex
The seamen of the Golden Circle suggest, when full of grog, that the women of Rebma are all beautiful and free with their favors. This is not exactly true, but it is not exactly false either. Rebman society in general is more cavalier regarding sexual relationships than its peers, but as in anything else, all Rebmans are different in their attitudes. In general, taking a lover (or two) is no cause for shame or scandal in Rebma, but the participants in such an affair are expected to conduct themselves with discretion. Flaunting one's lovers in public, or worse yet making a scene is frowned on by society. While a rake may be accepted or even welcomed in the Court of Rebma, a man or woman who occupies themselves solely and openly with the pleasures of the flesh will quickly find themselves near the bottom of the social pyramid.
Rebmans are long lived and not particularly fertile, while this cuts down the possibility of noble bastards, it does not eliminate the risk. If a Rebman is married, any extramarital affairs are expected to be approved of, or at least tolerated by the Rebman's spouse. Hiding a lover from one's husband or wife is considered a far greater betrayal of marriage vows, and on several occasions a favored lover may even find themselves a position in the household. Illegitimate children are not automatically stigmatized, but they are not permitted to claim family or inheritance without extensive legal wrangling.
