I have 2006 bayliner 205 with 5.0 mercruiser. The boat has been running great till this weekend. There were 5 adults in the boat and everything was fine till I had to stop. We sat around in a cove for about an 1 1/2 hr. The boat started good but when I gave it gas it would only go 5 mph. It would run like this for a while before picking up full speed. It would continue to happen the rest of the day. What could the problem be?