Check it over before you store it away for the summer. If it has holes, repair them. It it needs stitching, get it done early. If it's shrink wrap, make sure the marina recycles it.
Spring is the time to check and repair bottom blisters (fibreglass), cracks in the hull-deck joint (fibreglass), corrosion or cracks (metal), and soft spots (wood).
If your bottom paint is no longer repelling critters and weeds, you need to add a coat of the same type of bottom paint (paints that are not compatible will not adhere). If the paint is no longer doing its job or is falling off, you will likely need to scrape off the old paint and repainting. Do not paint your anodes, ground plate or instrument surfaces (knotmeter, depth sounder). They need to be kept clear!
Removing toxic bottom paint is dangerous to both you and surrounding environment. Use a face mask to keep out the paint dust, and wear clothing from head to foot that you can wash separately from your other clothing. The safest method for your hull's gel coat is still manual scraping using a putty knife. There are abrasive methods and chemicals that remove paint, but they can easily damage the gel coat, which is the very thin outer layer.
Change the oil, or at least check the level. Check or replace spark plugs, ignition wires, distributor cap, belts, hoses. The first year you own a boat, hire a good mechanic to do the work while you make notes. One missed engine plug can leave water inside the engine block.
Poor connections are common in the marine environment and cause intermittent operation and early failure of everything from light bulbs to electronic equipment.
12 Volt system:
Before connecting the batteries, you can safely open and clean the connections in your electic distribution panel. Clean battery posts and wire fittings with steel wool. Top up with distilled water (unless sealed) and charge them fully (over 12.5 volts). Reconnect the cables to the batteries and test the voltage with a "load" such as 20 watts worth of lighting to see if the voltage is maintained. Under 12 volts is considered time to recharge and 11.5 volts is considered "dead". Allowing the starting battery to discharge below 12 volts will shorten its life.
12 Volt equipment:
Test vapour alarms. Test bilge pump and automatic switch.
120 volts system: Check your GFCI (ground fault circuit interupter) plugs - they can save your life by detecting stray currents or malfunctioning appliances. Buy "rough" use light bulbs.
Empty all non-toxic antifreeze from the fresh water tank and lines. Partly refill and flush again. If you wish to sterilize the water system, add a small amount of bleach (1 capful to 30 gallons water) and fill the tank - pump the solution through all hoses and leave for a few hours. Flush and refill with fresh water. Bleach is not fish-friendly but this small amount should be exhausted if there is any bacteria or fungus in the system.
[I read in a magazine some years ago that food-grade hydrogen peroxide is an effective water purifier, and even had health benefits. I have never found a source - or indeed anyone who had heard of it. Pat]
Check toilet operation, hoses, fittings.