Don't walk there.
Specifically, don't walk towards the end of the spit at the north side of the river so as not to disturb the seals and ground nesting birds. There are plenty of signs on the trails from the Forvie Car Park alluding to this.
If you want to have a good look at the seals, park at the car park near the Newburgh Golf Club on the south side of the river just as you come into Newburgh from the south, signposted "Seals" Or park in the village and walk along the track as the car park if often busy. Then walk along the beach. Take binoculars and lots of photographs. You won't disturb the main body of seals 100 yards across the river and you will easily see them even without binoculars. And there are always a few curious seals in the water keeping and eye on folks on the beach.