While specifically not part of the actual sea trial if the boat is coming with a trailer this is an excellent time to get an unobstructed view of the trailer. The bunk padding will be fully exposed as well as access to the brake lines and brake components. Also check the brake fluid in the reservoir for water and condition.
I know this sounds a bit off topic for a sea trial but in my mind the sea trail should include all aspects of the package you are buying. I've helped several of my friends buy boats and this is an area that is often overlooked. Everyone gets all excited with the boat ride then overlooks the trailer. I got bit on this with my 95 200VRS. Boat ran great, I did the transaction and pulled away and at the first stop light realized the trailer had no brakes. I ended up having to rebuild the entire brake system to the tune of about $800. Would have liked to have used this to bargain more on the price.