I did some work on my boat recently (impeller, thermostats, carb cleaning, etc.). Motor is a '04 Yamaha Vmax 150 HP. (FYI, I ordered a new fuel filter and will be replacing that in a few days, just to keep up proper maintenance.)

For the most part it runs fine, but... I think it is idling too low as it will die from time to time while just idling on the lake, or after just getting to warm up after initial start. It never dies while actually in gear and running, just at various times while idling. It is very quiet in the water, but it just sounds like it idles too low. How do I go about setting it to idle properly? Do I need to adjust as per proper RPM setting, or sound, or what?

Believe most go by RPM. Should going off memory of what I have read here before but think it should be in the ballpark of 650 rpm. If you initially set it on muffs, with no back pressure on the exaust, that is probably why it is low in the water.

Might be time for Low Pressure Fuel Pumps also.