sandman is one of those characters who tend to suffer from a lot of pis. he has weakness to water--sometimes, to varying degrees. sometimes things phased straight through him, sometimes not. different things like being blown up have dispersed him, other times they haven't. he can be impaled sometimes, but other times laughs it off.
he's a character who is much more powerful in a forum setting than in a comic. that said, aquaman could beat him, like namor could. but he's a difficult opponent in a forum against someone who is mostly one dimensional. if namor and am have hydrokinesis available, they'd win, depending on the setting. tp would also be an auto win i think. lightning could turn him--or parts of him to glass. so there are definitely situations where am would beat him. if this is brick-style am, and it's brick style namor, it would be a lot mroe difficult.