In 1985 I bought a 15 mm lens that was used a lot until 1992. Then it was put away until 2011, when my son used it on an old Nikonos V during a trip to the Red Sea. That lens had never been serviced. The knobs worked smoothly, the pictures were in focus, and there were no leaks during the 40 dives he did.
The 15 mm lens is very complex. If it seems to be working OK, then I think there is less risk in taking it on a dive than there would be in having it serviced by someone who may not have worked with one for many years. Do you know anyone who could be trusted with the job?