I have a 2002 triton tr 20 with a merc 200 efi. this weekend a new wanring noise went off and was wondering if anyone could tell me what it means. It was 4 beeps and happened about every 5 min. it happened at idle and while on plane

Low oil in small reservior under engine cowling.

What causes this and what is the solution?

If its what Mike said you can take the motor cowling off and find the on board oil reservoir. It has to be full or the warning will sound. I don't know why they go low but I've heard that before.
I do know that you shouldn't put the cap back on too tight. They will crack.

The large reservior in the boat is under pressure. Oil will obsorb air and slowly deposit it in the small reservior. The small reservior is under a lot less pressure than the large tank is. So over a period of time, an air pocket forms in the small reservior.

Had this issue.... Just hook it up to the ear muff and let the boat run at idle in neutral then remove the cap to the small oil reservoir until the oil fills to the top then tighten the cap back down and turn off engine... I have had to do this on the lake also! Easy fix...

thanks guys for the responses