This early entry to the Carry On legacy doesn't seem to fit the bill for many people. Maybe there were too few of the core team - no Sid James, no Kenneth Connor, no Hattie Jacques. Perhaps it was that this was the first attempt at an historical setting, rather than the contemporary, work-place films that had kicked off the Carry Ons. I'm not sure anyone really knows.
Let's consider the evidence.
First the plot. It's around 1800, and naval hostilities against Spain are at their height. Britain needs every man available to crew their warships - even one as inept as Albert Poop-Decker (Bernard Cribbins). Having failed to graduate from Naval Academy seven times, they really can't wait any longer, and decide to award him his midshipman rank and posting to His Majesty's frigate Venus. When he gets there, he falls foul firstly of a local girl called Sally (Juliet Mills) who wanted to go to sea in search of her childhood sweetheart, and secondly of the press gang, led by Mr Howett (Donald Houston). He wakes up in the hold of the Venus, next to local cesspit cleaner Walter Sweetley (Charles Hawtrey). It's not long before he things are made clear to him: his place next to Captain Fearless (Kenneth Williams) has been taken by Sally in disguise, and his days consist of staying clear of trouble as a lowly seaman. Until there is a mutiny, and it's time to stand up and be counted - both as a sailor and as a man in love....
Now this story has promise, and indeed some of the stars deliver. Williams and Hawtrey are both very good at doing what they are very good at - delivering a line with the right amount of innuendo, timing, and pathos. But once you get past their performances you tend to see the cracks. Cribbins is OK, but in truth he's far more convincing as the downtrodden victim than the upstanding hero he's forced to become. Juliet Mills is.... pretty. Which isn't a quality you'd want in a midshipman (well, not unless you were on an excessively long voyage where even your bunkmates began to look attractive). As a pretend man, it doesn't really work.
Most of the action takes place on board ship, which is a little confining, and leaves too much emphasis on actors whose performances were below par to begin with.
All in all, whilst there were certainly worse Carry On films (especially towards the end of the series), this was not one of the best efforts, and certainly would have been improved by more of the regulars.