Take I-5, either southbound or northbound depending on your origin, to the Kent-Des Moines exit, Exit 149. Head west all the way down the hill some 3 or so miles into the little townsite of Des Moines. Go to the water. Big ole marina. Go to the far north end of the marina. You will see the fishing pier. Just to the north of the pier and parking lot is the park. Thereyar.
Alternate route from the north - frequently better for traffic (may help to check a map to see how this works):
Take highway 509 south, along the WEST side of SeaTac airport. When the freeway portion of 509 ends, you have two choices, which work equally well:
1) Go a short distance east on S 188th, taking an almost immediate right onto Des Moines Memorial Dr, heading south. Follow Des Moines Memorial Dr. until it T's with S 216th / Marine View Dr. Turn left, heding south on Marine View Dr. into the Des Moines townsite, then head west to the water and marina.
2) Follow signs to continue along route 509. This will cause you to go west on S 188th, then south onto 1st Ave S. Continue following 1st Ave. S, which turns onto S 216 St., then Marine View Dr. into the Des Moines townsite.