We must be talking about a different Utila! The Utila I know doesn't have any rivers, only swamps and lagoons. It's too small to form a river. We also have no beaches where you can do a good dives from.
OTHO If you are down on the Cays there are a number of great shore dives to do, if you are already on the Cays! You need a boat to get to them and you need a boat to move between them, but you can enter the water easy enough from all of them.
Most of this Utila has fringing reef right up to the shore line making shore entries difficult. Where this isn't the case we tend have fossilized corals and volcanic rock making walls that are impossible for shore entry. But the trickiest thing of all is that 90% of the islands shoreline is inaccessible from the town.
Having said all that, if you know the right places, or you are staying at one of the resorts there are some great shore dives. Deep Blue and Laguna Beach resorts both offer shore dives from their own docks which make access much easier. Airport Reef from Lighthouse to Rocky Point and even further round to Redcliff is accessible and has some great dives, Redcliff noted for it's sqim throughs, Rocky Point has a real nice deep dive, Ted's Point is always pretty, and the drift from Ted's down to Moonhole I already mentioned (my personal favorite!). You need to be careful of critters, corals and yourself when entering and exiting.
-rob