The "at sea" part might seem rather redundant, but I suspect that the author is playing a pun on the words "海上" and "会場" (meeting place, grounds) which both carry the same phonetic reading of "kaijyou" as it is written here in hiragana. It could very well be a promotion of sorts for Natsucomi or something similar, but since I'm unclear on the artist's intent, I opted for the literal context.
Maybe someone more in the know could clear this up? :)
