’spotted’ in this context means ‘seen’ or ‘witnessed’. ‘Scurry’ is a verb meaning to move quickly, I think you’d mostly see this word referring to the way that rats or mice would move. I would therefore assume that it means that british crewmen had been seen moving around (the use of the word scurrying makes me think theyre moving suspiciously, I would need more context to know for sure) the boats tethered beneath the HMS somerset. Hope this helps :)

To spot dsomething is to see it, to notice it. scurrying is moving about, usually hurriedly, sometimes furtively. . I spotted the cat scurrying round the fride. Sure enough, one of her toys had rolled under there.