Outside electric outlets should be 20amp or 15?
You could probably use the 15 ampere basement circuit to power outdoor lighting, and water feature (just make sure it's all GFCI protected). If you're going to be using power tools outside (power washer, electric mower, trimmer, hedge clipper, etc. Then you're going to want to run a new 20 ampere GFCI protected circuit.

If there are only a few lights in the office on the circuit, you can surely power a few more garage lights with that circuit. Though you'll want to make sure that's all that is on that circuit, or you could end up overloading the circuit.  If you want to add receptacles in the garage, you'll want a dedicated 20 ampere GFCI protected circuit. If you're going to be running a bunch of power tools, you might want to consider a 20 ampere multi-wire branch circuit. In which case you can split each receptacle, and power the top/bottom from the separate legs of the circuit.

Before you make a final plan, make sure you know for sure what's on any circuit you're tapping into. Guessing here could lead to overloaded circuits, and nuisance tripping.