My HRM Swim Blue Surround has cracks around the edges and needs to replaced is that part available. Seems to be bad material used for this. Next I swim it will probably fall off and only be relying only on the internal seal.
The blue seal doesn't really do anything useful besides smoothing out the edges. The o-ring on the cover is your actual water proofing. I dont know if they sell the part but you can try using the chat on the garmin website. I have had good luck finding random small parts I have needed previously like the speed sensor casing and the specialized alley key for my Garmin a1i pro sight