Roman legionnaires were taught to swim with their Lorica on. It was probably not very practical but Roman training emphasized physical training and moving around in full kit a lot meaning any Legionnaire would have been able to swim around for a while should he fall into water during a boarding.

Vegetius mentions that among the skills Roman soldier had to have was to be able to swim should the necessity arise (pursuing enemy). He does not specificate whether this is with armor on or not but the fact he presents this as a skill necessary in case of emergency it can be understood that the soldiers would probably not have the time to take off his kit before crossing.

Egyptians like a lot of armies from hot countries would generally fight in light armor, the officers and higher ups would often wear light armors made of leather and copper, the rank and file would wear light leather or just an armour made of reinforced fabric so swimming in these would probably not have been an issue but I am not sure they would board a lot. It was more a specialty of the Romans who went a long way to transfer their land based advantage (highly advanced infantry with high training, morale and discipline) into sea warfare that was initially more a Greek specialty.