Any clear finish that I know of is going to darken teak. So will oil. If you are sanding interior teak on bulkheads or floors, it's probably veneer, so be careful not to sand through it.
One thought. Was the wood previously varnished or oiled? If it was oiled, oil attracts dirt over time, so a cleaner and brightener might lighten the wood a bit before a clear finish is applied. However, those products require a lot of rinsing with water. A messy job to attempt inside the cabin, especially on vertical surfaces. You also risk ending up with uneven or blotchy results with the brightener. Might not be worth it.
Personally, I would just go with the clear finish and learn to love the richness of the darker teak. You can lighten the overall look of the cabin with light color cushion covers, shades, etc. so it's not oppressively dark. That's my 2 cents.
Whatever you do, we like to see photos here, so please share some before and after shots. I'm sure it will look great.