These are not the same 'brand' of box jellyfish that inhabit Australian waters and can kill within minutes after causing such intense pain that the victim wishes death would come instantly, so I wouldn't be scared out of the water completely.

The big risk from these critters is just a nasty sting, nothing life threatening, unless you have an allergic reaction. If it were me, I would use caution and swim at beaches with lifeguards who are able to determine if jellyfish are in the water and post the appropriate warnings. If you do get stung, the lifeguards should have a kit on-hand with some basic remedies.

FWIW, I have been in those waters during those periods of time completely oblivious to the jellyfish and was never stung, nor did I even see one.

Good beach snorkels can be had at Black Rock (Kaanapali), Kahalu'u Beach, and Honaunau Bay (Kona) - Kahalu'u has a sheltered breakwater and might offer more protection from any migrating nasties. Just check the condition boards posted at each beach before diving in.