Sanji knows Luffy is a Devil-Fruit user from witnessing the SPLOOSH on page 9. The Anchor Boy nickname verbally reactivates the Ch. 1 Anchor shirt prop-tell.
The payoff
Sanji is the second future-crewmate to publicly call out Luffy’s DF-weakness (after Nami’s Ch. 8/10 easy-prey register). The early-articulation pre-loads the Sanji-is-Luffy’s-water-fail-safe dynamic — Sanji will save Luffy from drowning across many later scenes:
Arlong Park (Ch. 89): Sanji dives after Luffy when Arlong dunks him in the pool.
Davy Back Fight / Foxy’s (Ch. 305+): Sanji’s underwater rescue routines during the Groggy Ring competition.
Marineford aftermath (Ch. 580+): Sanji aids in Luffy’s underwater extraction.
Fishman Island (Ch. 605+): Sanji as the only Straw Hat able to swim-fight at depth.
Whole Cake (Ch. 870+): Sanji’s underwater navigation skill during the wedding-cake escape.
The Ch. 53 Anchor Boy line is the operational prologue to Sanji’s later water-side rescue role.
Why this matters
Oda’s nickname-as-foreshadowing technique. A character’s casually-deployed nickname for another names the dynamic that will recur across the series. The nickname is immediately funny (cf. Mosshead for Zoro, Curly-Brow for Sanji from Zoro) and long-term predictive (the role-relationship implied by the nickname will recur). Sanji’s Anchor Boy names a dynamic — Sanji-saves-the-sinking-Luffy — that he will operationally fulfill across the rest of the series.