Issues with 03 Yamaha 4 stroke 25 hp

Bought a boat with a 03 yamaha 4 stroke 25 hp. When I looked at we hooked it up to hose ran great in yard fired right up sounds good plenty of power. Take it out this morning for first time. Fired right up and sounded like a brand new one. Give it throttle and it had no power at all. Ran it around cove there hoping it was just gummed up a bit and still think it probably is. Bring it home put it on hose fire it up and it revs right up in hear and sounds like has plenty of power. Decide to change oil while I’m home. Noticed old oil was a little milky but not horribly bad. Go ahead and change oil put some seafoam in gas and let it run a while at idle hoping carb is just a little gummed up. Take it back to lake and still same problem did seem to be a little improved so I’m sure power issue is in carb. But now the part that has me concerned get it home and oil is milky again. I’m thinking head gasket but I’ve never messed with a 4 stroke outboard at all. Anybody have any idea where I should start? I’m thinking head gasket maybe?