If its the V-Drive, more than likely be more than a seal. If you are leaking fluid, there is probably pitting in the transmission output shaft.
V-drive: I am not sure what year boat you have, but if it is an late 90's-early 2000's V-Drive then you would definately benefit from doing a complete service (both front and rear seals). These year boats only used single-lip seals and are able to pull water into the transmission when it is running. Go ahead and install the double-lip seals. As I said the pitting on your shaft will require a speedy sleeve which will give the seal a smooth surface to work on. This process requres a transmission pull, and somewhat labor intensive, but it's the best route.
Whether it is a V-drive or direct drive, you need to look at your sealing surfaces. There is a reason why there is a leak.