Good luck with the 424....nice boats!
Yes... you do need a cover...the plastic shrink wrap works just fine. No cover means ice and flooding in the cockpit and water below aside from the protection the cover affords to your fittings and fiberglass and teak.No...you cannot keep it in the water...water in many hrbors freezes 2 feet thick at times and can crush your boat and spring through hulls and sink it. COMPLETE winterization of all
lines, heads, engine etc. is necessary to withstand -20 temps!
Go on the Pearson e-mail list or owners group and ask others what issues came up on their surveys so you can better guide your surveyor on problem spots to look for. Make sure you get a diesel guy to test the engine & compression in water before haulout as you don't want any surprises come spring and most sailboat surveyors are NOT diesel guys!
Good luck!