It is a great spot to start just out of Open Water. Utila is all about diving and they will match you up with a DM and you will learn lots about diving by the end of your trip. Everyone helps each other out - divers all levels - it is like a dive family and everyone will look after you.
Laguna Beach resort I just met a DM there on Bocas. He said in Jun then saw 2 whale sharks there on Utila.
The water is warm however the more dives you do your body temp will drop, so as time progresses you may get a bit colder. Just don't even bother packing one - just rent one when your there - it is cheap to rent and don't have to worry about packing a wet one.
They dive all over based upon what dive sites are good - not much swell - current and places with good vis!
Whale sharks is a hit and miss - maybe they report them if they are around - but keep in mind - whale sharks are seen year round but for the best chance of seeing one, we recommend timing your visit during the most active months of March/April/May and August/Sept/Oct.