It all depends upon what you are planning on doing. I am a huge fan of the Teva type water sandals, they give you support if you are planning on doing any kind of climbing/walking or if the beach is rocky. 1 pair of these water sandals would replace flip-flops and tennis sneakers. The reason tennis sneakers are suggested is for climbing places like Dunns River Falls in Jamaica. You can't wear flip-flops doing this, you would seriously hurt yourself. The tennis sneakers give you just enough grip and support and they dry fairly quickly after getting wet.
Lands End has a new type of shoe, styled like a sneaker, but designed to go in the water and dry quickly when out. Also has an adjustable plastic thing to tighten them, rather than tie. I'm thinking of trying a pair, they're only about $25.
If you are talking about walking any distance on sand, you need something that will fully grip your foot. Flip flops don't cut it on sand. Even if they stay on, you get sprayed with sand on your back as you walk (very often). Trust a Floridian: Flip Flops are for wearing on a solid surface - NOT sand!
Sandles are your best bet (since they don't fill with sand), but any sort of actual shoe will work better than flip flops.
And avoid flip flops completely if you plan on any sort of activity beyond walking (running, climbing, etc.).
On many of the CAribbean Islands, be prepared for the walk off the pier and then to the taxi stop--and then, at the beach destination, off the road and down to the beach. Also, be prepared for a very wet and dirty beachside restroom where you would endanger your health with bare feet. We have often encountered rocks, broken concrete and occasionally broken glass on the way. We all wear some sort of shoe and carry some sort of flip-flop for the sand. The water shoes are a must for dunn's river falls and the slippery surfaces.
I just try to plan for all contingencies--we have seen people with cut feet, all bandaged, after a beach excursion and it looks miserable.....Of course, if all is lovely, and you have taken all precautions, that is fine too!