Take the second level of human form (involuntary change +2) on Mythic Strength and have the change depend on how much energy he absorbs. As for his physical immunity, his catch should be "non-energy" for +3. I.e. cold damage, energy drain, acid damage, poisons, entropy magic, vacuum, suffocation, void magic and similar hazards don't impart energy on the target at all - not even kinetic energy - and some in fact kill by draining energy instead. That makes him a nice submerged character cost-wise.
As for other X-men, you don't have to limit them to submerged. And once you do not, they are all stattable. Professor X for example should have Senses -3, all used for his level of Telepathy (that kind of sense is equivalent to a dozen individual supernatural senses), Incite Obedience -4, Master Dominator -4, Unlimited Mental Range -3, Area Mental Power -20 (to be able to affect 10 zones at once without penalty), Speed of Thought -2 (can always act first initiative-wise as if he had Mythic Speed, limited to mental actions). This puts him at a whopping -36 refresh but makes him capable of instantly controlling an entire stadium of people, permanently rearranging the thoughts of others, reading thoughts and emotions as easily as we read roadsigns and so on.