This marshy area is about 2 blocks from downtown Martinez, just cross the railroad tracks and you're there, right on beautiful San Pablo Bay... There is also a marina with a few piers and you could kill an afternoon walking around Martinez downtown and the coast area right there. You could, if you were really bored. Firedancing4life has pictures of the new bridge going up in the area. Yelp has a review of the John Muir site also in the area.