I mean, Superior has the world's longest freshwater sandbar.  All you need to do is drive down to one of the intermediary points between PP and the airport and yer golden, if that doesn't work, head over to Wisconsin, there is never a need for anyone to want for unpopulated beach around here, there are miles and miles. In the immortal words of Buckwheat, don't go lookin' pa nub in all the wrong places.

The mouth of the Knife River has a moderate beach.

We go to Pike Lake out Martin Road near the boat launch.  It is never crowded and the water is usually calm and shallow.

Knife River Marina beach (I think different from the "moderate" one at the mouth of the river.

Turn right at Knife River Campground off Scenic 61, cross railroad tracks. Veer left off the paved road onto gravel, follow straight to the beach.  It is marked once you get there.

This has been a recent favorite for me and my little ones because: a.) no crowds b.) short walk c.) agates d.) Great Lakes Candy Kitchen in Knife River. (Across the river on 61)

There isn't sand, necessarily, but tiny rocks, which are still apparently entertaining for a 3 year old.  Also some big rock shelves for climbing on.

Two reasons not to swim on the 'wrong side' near Sky Harbor Airport:

1. It is the St. Louis River Bay = polluted. Though water may be cleaner than a decade or two ago I am very suspect of what lies within the soils at the bottom of the bay and what gets stirred up when they perform the needed dredging to clear the shipping channel.

2. That little bay within a bay is actually a seaplane (aka floatplane) landing area. The faded signs scattered about attest to this and state no "watersports". Obviously much ignored by local swimmers and boaters. Depending on the wind conditions airplanes may actually use that area for landings and take-offs.