And there may be a thousand other reasons - including that the odd object was only noticed (if at all) as the boat drew away and there was no chance to go to it easily (surf, wildlife there, etc.).
Reading Bevington's journal and listening to him (on the DVD, recorded when Ric and Pat visited him more than 50 years later) recount his journey around the entire island, I'm of that persuasion, Jeff -- "that the object was only noticed (if at all) as the boat drew away."
Their expedition approached the island from the SW that first day and tied off to the
Norwich City, which he explored. He crossed the reef, then the Tatiman Passage. His circling of Nikumaroro proceeded counterclockwise, and on the DVD he says that his group may not have finished their lengthy hike until the next day (camping overnight somewhere -- along the northern shore?). Whether the trek went entirely along the shore, up and around the NW corner, or took a path along the lagoon shore to get back to their starting point, is not entirely clear to me. But there's no doubt that thirst was playing a big role by that time, and even if the artifact we call "Nessie" was noticed at that time, I'd bet getting back to their fresh water stores would have trumped any inclination to investigate that ("I ... drank 4 pints of water").
The remainder of the stay on the island sounds like it was very focused on exploring the land and seeking a source of fresh water, and not looking at things like offshore curiosities.