Wiring the out building
That supply cable is nice and stout and can support more than 60A.  I agree with R.M.E... Whether you'll need that depends on the size of the water heater.  The can lights are of no consequence, and the deciding factor is what else you'll be plugging into the 9 sockets.  

Regardless, I strongly recommend "going a bit nuts" with the subpanel size.  You want a lot of extra spaces, because running out of spaces is a serious problem so easily avoided!  Aim to have twice the spaces you expect to use. You figure "oh, I'll just run a table saw", 1 breaker space, then you find out you can't run a dust collector on the same circuit, +1 space.  Then you get a nice deal on a 220V table saw +2 spaces, etc. etc. etc. 

Also, don't get a 60A subpanel because you won't get full capacity out of your wire.

The subpanel can be larger than the feeder breaker and wire. Get at least a 100A because your wire can support it, or even a 125A if that makes sense (bigger panels also give you more bonus breakers).