The Mk4 (junglie) Sea Kings don't have retractable gear, on the Mk 3/3A (RAF SAR) the AVADS warning goes off at 30' if you don't have the gear down so, when winching with very small vessels (low to keep visual references) we often drop the gear.
Other than that it is up to the pilot whether he has gear down or up but it makes no difference to deploying the flotation bags and, apparently, does not affect the water entry on ditching either (that from the waterbirds instructors).
Overland at low level it is eminently sensible to have the gear down (one less thing to forget if it all goes quiet) and I am sure I am not the only one who has forgotten to raise the gear - (yes I know it's in the checks)
or lower it!