I applied Minwax satin oil base poly to any teak on our R23 that might get wet or dirty, such as the walls near the sinks, the bathroom/shower door, etc. One coat adds some water protection and some shine. A second coat increases the shine, makes it more even and consistent, but might start to look too shiny compared to the teak that doesn't have the poly. I didn't want to poly all the teak because if it fails after years of sunlight coming in the windows it would be a lot of work to redo. So far It's been 2 years and it looks great.
For prep, I sanded with 400 grit and gave it a good wipe down to remove any sanding dust. One caution, it can be hard to get an even finish with poly using a brush. I have a lot of practice from my work doing carpentry, cabinetry, woodworking.