Funnily enough, https://americaexplained.wordpress.com/2011/01/22/running-jo..., claims to explain a joke based on the Asterix simulacrum of "Le radeau [...]" but it is an entirely different joke and no mention made of the inspiration for that image. Someone calls the situation out in a comment.
Both are true. The pirate ship being sunk is absolutely a running gag in the Asterix series, and in fact I had a vague recollection that the raft scene also appeared more than once, but once I took a closer look, that appeared to be wrong. So, lots of ships of this particular band of pirates being sunk, and once they are posing on this raft in a classical way.
