There isn't a lot to do on a Merc powered boat, since the engines are self draining. But... here's a basic list.
1) Put fuel stabalizer in the fuel, and run it into the engine.
2) Clean the boat, and make sure it dry. (inside and out)
3) Use fogging oil in the engine.
4) use a protecting oil on the engine, and other parts in the engien compartment. (Or silicone spray)
5) Make sure the Potty is clean, and empty.
6) Use an RV anti-freeze in the fresh water system, and pump it up to all the hose ends. (the sink, shower, and water tap in the steps)
7) Lube all the external pump parts. (nozzle pivot, reverse bucket, and cable ends) Realistically... just using the protection lube on all of it is OK. But, the pivots should be greased if you do the pump disassembly.
This is the hardest step:
8) Service the jet pump.
a) At minimum... drain and replace the oil in the cone, and gear box. (use only Mercury high performance gear oil for sportjet's. NOT normal gear lube)
b) You should disassemble the tail of the pump, and verify the wear ring, impeller, seals, ect. But... if the lube is clean, it't not mandatory.
EXTRA:
You can also drain the vapor separator, and fuel filter. Or better yet, replace the spin on filter.
IN THE SPRING:
1) Put water, and chemicals in the potty.
2) Drain the anti-freeze from the water system, and flush with fresh water.
3) go enjoy the boat.
EXTRA:
after the first ride... I recommend putting in fresh spark-plugs.
(this list may update, ans I remember things)