I believe the short answer is that you should get one of the protective cases that are available to protect against sand and dust but only rely on the water resistance rating as stated by Tandem:
“IP28” means, “2”, that it isn’t particularly resistant to solid objects; it is not resistant to sand for example. The “8”, however means that it is protected against immersion in water deeper than 1m (3ft) under conditions the manufacturer specifies. The above web page states the limits:
The Tandem Mobi insulin pump is water-resistant to a depth of 8 feet (2.4 meters) for up to two hours (IP28 rating) when the cartridge is loaded.
The spec sheet of your mobi should also include the IP28 rating along with the limits for the “8”; IP28 is meaningless without the manufacturer limits.
The limits are longer than the Omnipod (1 hour) but less deep (25ft for the Omnipod), however neither device states the result of repeated immersion. I know from experience that the Omnipod will fail within an hour as a result of repeatedly diving under the water, even to a depth of less than 25ft.
I’d get a protective case for the mobi simply to protect against sand and dust but I can’t see a way of protecting a mobi or any similar insulin pump against water beyond the manufacturer’s recommendation (8ft for 2 hours max). Tandem go on to say this (emphasis added by me):
The Tandem Mobi insulin pump it is not waterproof and should not be worn while swimming, scuba diving, surfing, or during any other activities that could submerge the pump for an extended period of time.
I.e. do not swim or snorkel for longer than 2 hours.
It should not be worn in hot tubs, whirlpools, or saunas, to avoid exposing the pump to high temperatures.
I.e. no water temperatures above 104F (the hot tub approved max, also a quoted max for skin temperature in a sauna). Notice that US hot water temperature is limited by default to 120F.
You may have to disconnect if you are going to be immersed in the water for longer than 2 hours however if you disconnect for longer than 1 hour you need, at least, a long acting insulin. For SCUBA I always swapped back to MDI; I kept the Omnipod on but reduced the basal to 0 (it lasts longer than way) but then didn’t replace it when it failed. It didn’t fail on training dives; shallow and mostly short, it invariably failed on recreational dives even if the dive stayed above the Omnipod 25ft limit.
I would avoid diving while snorkeling even if I stayed within 8ft but most untrained snorkelers are unlikely to even dive that far.
TL;DR:
I believe there is a way to build a dry-bag to enhance the water resistance of a tubed insulin pump. It is necessary to have some kind of sealing ring on the canula tube, e.g. an o-ring, but given that the tube can be temporarily disconnected and threaded through the seal I don’t think that is a problem.
Once this is done the dry-bag can be inflated to give enough air to support a particular depth or could be pressurized or, indeed, both. Then the bag will simply breath in and out round the pump and no water will get in. (This depends, as with any dry-bag, on the bag seals being correctly sealed!)
A more sophisticated solution would use a flexible pressure bladder within the dry bag which expands into a fixed space inside the bag. This automatically accommodates pressure changes without requiring careful inflation of the main compartment of the bag.