They will no doubt eventually get splashed or dropped into the water and need to keep working.
Waterproof electronics are rated with an IPXX number, for example IP68.
IP stands for Ingress Protection and basically means the ability to keep objects out of your electronics.
If a device can keep out water most will understand that objects the size of dust also wont be able likely to get in.
The second number is rated for Liquids and is the number most Bluetooth speakers will include in their rating.
For example, IPX5 or IPX6.
The rating for liquids go from 0 to 8 with higher being better.
IP Protection Numbers Against Liquids
IPX0.. No protection
IPX1.. Protection against drops of water vertically
IPX2.. Protection against drops of water when tilted 15 degrees
IPX3.. Protected from water falling as a spray
IPX4.. Protected from splashing water
IPX5.. Protection from water sprayed from a nozzle 6.3mm
IPX6.. Protected from water sprayed from a nozzle 12.5mm
IPX7.. Protected when immersed in water up to 1m
IPX8.. Protected beyond 1m of water submersion
A Bluetooth waterproof speaker should have a rating of at least IPX5, as it would be able to take a splash or spray of water.
A IPX5 speaker could not be submerged in water but could withstand some degree of water and survive.
Of course, higher the better and a unit with an IPX7 would likely be best as it could be dropped in water for a short period and survive.
Buying a speaker able to withstand water will depend on where it is going to be taken.
If you only plan on sitting poolside than an IPX5 unit should be fine.
If you plan on going tubing down a river than an IPX7 would be much better to get.
Even though these ratings are meant to give a good idea of what a speaker can handle other things such as the build should also be taken into consideration.
Top-End brand names with a reputation to keep usually will put more thought and testing into their speakers than a no-name brand.
The sound quality also will need to be taken into consideration and varies form unit-to-unit, as well as manufacture-to-manufacture.
As always be sure to read the reviews on a unit to be sure it is the right fit for you.