Thanks for your response. I should have mentioned that the pump was replaced prior to sale, so it's brand new. Apparently the yard that did that work though never checked to make sure the water system worked after replacing it.Your boat is 12 years old and has not been used in three years. Bet that the water pump rubber impeller and rubber valve assemblies inside the pump need to be replaced.
By all means, you can flush the lines to make sure that the lines are not blocked and to ensure there isn’t an air lock. Don’t bother emptying the tanks yet because you can drain, flush, & sanitize them once the pump is operational.
You will have to decide on rebuilding the existing pump or buying a new pump. The rubber impeller and rubber valve assemblies will cost approximately $80 including shipping; a new pump will cost approximately $140. Just went through this same issue a few months ago & decided to replace the oem Shurflo pump. If you replace the pump, be sure to buy one from the same manufacturer with same flow output to simplify the installation. Don’t bother with the expense of dealing with the yard, you can fix this!