Anthony Joshua clearly ain't sweating about whether his superfight with Deontay Wilder will ever go down ... 'cause it's kinda hard to perspire waist-deep in the Atlantic!!!!
The unified heavyweight champ hit Miami Beach on Sunday ... showing there's not a care in his rock-hard body right now about a possible AJ-Wilder tilt.
Joshua bro'd down with some buds ... splashed around in the water ... and toweled off on the sand -- all while everyone still wonders if he'll ever scrap with The Bronze Bomber.
Reports say it'll get locked in for next April .