look at the picture, clearly Zoro & Sanji is swimming without any problems, & they are helping Brook & Robin, because Brook & Robin can't swim. the only one whose clearly having problems here is Usopp.
Nami doesn't appear clearly in the frame, but she seems to be fine
I think he already ate a DF, but he doesn't know about yet.
I mean Oda deliberately showed us that Usopp can't swim, so I guess it's impossible for Usopp to eat Kuma's DF, because he is already a DF user