Was Law dumb or something like that?

You are one of the Luffy's crewmates, being the only one that managed to get closer to Law without him threatening you to death. But after your captain offered for them and the older black-haired's crew to spend some time on the beach, you couldn't not notice Trafalgar being all happy and excited, even if he was a devil fruit user.

Standing at your front, he was wearing a swimming cap and shorts, along with swimming glasses. His tattoos shined on the sun as the others played or chatted around you and him, he was smilling proudly. "How do I look?" You would expect that from Luffy... But I guess him and Law share a braincell.

...

How do you tell him that he cannot swim?