I think it's actually quite revealing that a Google search for Chikitu in combination with the words "safe", "diving" or "swimming" turns up practically nothing other than this thread. Could it be that no one lived to post about their experience or they just didn't want to talk about what happened?
"Playa Chikutu is beautiful, but, alas, swimming is forbidden due to the enormous waves and the dangers that lurk underwater. You would be swept off your feet right after you walked in due to a strong current that passes the island on the north coast." (Link)
"Once open to swimming, this beach which faces into the open swell of the Carribean Sea is now only for sunbathing after a man was sucked into the open water by the undertoe. One look at some of the giant waves that come crashing into the beach will normally be enough to convince most not to swim here." (Link)
"Playa chikitu (not boca), high waves, stay in the center, not deeper than your waist. Canadian boy drowned there." (Link)
And then there's this view, as seen from the parking area:
(Link)
I hereby retract my previous comment about "no need to post signs..."