There is always the option to go by boat. Supply vessels go out every day.
Bravo73, for information there are sea state and visibility limitations for vessel transfers just the same as for helo transfer. Over here in the Grand Banks it's about 3 metres and the crane operator has to be able to see the PAX when transferring them in a Frog 3 at a time and/or a Billy Pugh.
In most cases the green light is given for helo transfer long before vessel transfer is an option. Only over here where we can end up with nearly two weeks without flying due to fog do we resort to vessel transfer on a somewhat regular basis. The only other time that happened was post-491 when the S92s were all grounded.
I don't know how many supply vessels you have these days in the North Sea, but when I worked over there about 9 years ago some installations only got a boat once or twice a week.
Sea states change quickly, so after 12-15 hours on a boat getting out into the field, you can sometimes end up "boomeranging", not a pleasant experience. That is why operators and PAX prefer the helo transfer option. However, all personnel transfer has to be made as safe as is reasonably practicable. Flying a/c with suspect QA/QC issues in a specific number of high priority components, i.e. bevel gear shaft required for MRGB lub oil system, does not IMHO meet this requirement.
Everything we do has inherent risk incorporated, how we choose to minimize that is up to us.
Safe flying
Max