With regards dive operators, recoveries onto boats etc; liability could be covered by a disclaimer that the diver signs? Something to the effect that they understand and accept that being overweight can make recovery/aid problematic and/or impossible.
With regards personal choice, I see no problem with it. Your life, your choices.
With regards your choices, they shouldn't effect someone else. If I herniated a disk pulling someone overweight from the water, I should be able to sue them. Or I should be absolved all responsibility for pulling them from the water.
In Europe, at least, there are occupational health and safety regulations that cover issues like lifting heavy objects (manual handling). Humans can be heavy objects.
Regulations state that employers must assess the risks to their employees health and safety from manual handling tasks and take measures to reduce the risk of injury so far as reasonably practicable.
That might mean fitting pulleys and slings to dive boats to pull overweight divers out of the water. The costs of which should be covered by the customers.
If you cannot eliminate or mechanise the manual handling tasks, you must carry out a risk assessment where the task could present a risk of injury. You need to look at ways to reduce the risks to as low a level as reasonably practicable.
That risk assessment, in the diving rescue/recovery context, could lead to rescues not being effected on overweight individuals. It should provide reasonable protection against litigation for not effecting a rescue.