I took my boat out today and the engine ran fine but when I started out across the water it started to lose power from the engine to the lower unit. It seemed to bog down after a couple of hundred yards and started to slow down. At idle speed it was OK but could not get it to go any faster without it bogging down. When I shifted back into neutral the engine revved up fine. Can anybody tell me what I need to look for to resolve this problem. Thanks in advance for any help.