Idle and Max RPM

[Reel Insane]

I run a 2007 150 txr on a Century 2001 a 20 goot vessel. Last trip out I noticed that my idle was around 400 to 500 after it warmed up and at WOT I was 4600 to 4700 trimed out as much as I could maybe 4800. It was windy out, but in the back water I never had this problem before. When I got home to flush out my engine I noticed that it idled just fine (700 to 800). Does being in the water have that much effect on idle? I never thought that there would be that much back pressure on an engine with it in the water. Also my engine has never turned over 5000 rpms. It is that fine? I tried to prop my boat to save a little fuel as I don't care how fast I go in the ocean. Any performance advice would be greatly appreciated.

Thanks in advance.

[boats.net]

If the model number is 150TXR(carb model) The rpm range is 4500-5500 rpm's and should idle at 700 rpm's in neutral, 600 in gear.
For best fuel economy prop to high side of rpm range.