Yay the shark!!
Like many I suspect there was no fight. Either the Polar Bear died from exhaustion after swimming too far/long or was injured somehow else and the shark fed off the recently deceased bear (thus no bugs as it was soon after it deied).
Fresh water is, as you say, not much safer...
Bull Sharks (can reach about 3.5m) are often found in rivers/freshwater
as are Johnsohn Crocodiles (grow to about 3m), Alligator Snapper Turtles (about 2 - 2.5ft), some venemous snakes...
Best to stick to the bike, not swimming.
