It's probably still better to train all the pax in HUET if you are going to fly them over the water - it works in O&G - just depends how much you value your pax vs what revenue they generate.
The fundamental difference is, the O&G SLF are going to work and have no choice over their mode of transport. So 3 over-water flights mandates they have a HUET tickets. Somebody like a tourist going on a jolly over-water one time only does have a choice, they can choose not to go, or perhaps choose which operator to go with. A pre-flight briefing and an on-board safety briefing card should be the limit of the requirement for them (in a language they understand).