If you have found Sky Haven Temple already then start by fast travelling there, then head South-West towards the river, once you arrive get in and swim until you find a small canoe. Within this canoe you should find a few gems. Gems being Rubies, Sapphires, emeralds, garnets, or diamonds. Flawless or not. You should also find a strongbox on the boat and under the water around the boat a chest.
Depending on your speech level you can fetch a pretty good price for them or you could get some gold/silver ingots and create jewelry at a forge to increase their value.
Added by: nightmare2 Jun 29th 2012, ID#4721