Bismillah

Al-jawab billahi at-taufeeq (the answer with Allah's guidance)

A husband and wife may satisfy the desire with each other in lawful ways as often as they desire. One may see one another nude. However, in all acts of intimacy, the spouses should exercise shame and modesty.

It is reported that Prophet (sallaallahu alahi wa sallam) had a bath with Aaisha (ra)-although this was without them looking at each other’s Satr.

Also, husband & wife should refrain from bathing with one another when other people can see them, or in a place where it is most likely that other people may see them; as bashfulness is one of the traits of faith, as reported from the Prophet (sallallahu alihi wa sallam). (Al-Bukhaari and Muslim).

So, one should observe the Islamic etiquettes as sunbathing is the practice of the non-Muslims who have no honour nor dignity nor bashfulness.

And Only Allah Ta'ala Knows Best.