We made it. Found both the caches and hiked around Blake Island. What a fantastic day of caching!!!!!
We launched here (Google map). There's room for one vehicle to park here without blocking the gate. Don't block the gate!
From here it's just a little over a mile across the water to the Blake Island South End Campground. There's a trail leading up off the beach at that spot. We found a father and daughter canoe pair camping there.
We planned our trip for a day with a low tide just after our crossing. This gave us a favorable current in both directions. We also selected a day with good weather, high pressure and little wind. We found the water almost glassy for the crossing. We really lucked out with the weather.
The inland water advisory for the area predicted 1 foot swells with winds up to 10 knots. The tidal current, the swells and the winds were essentially a non-factor for this trip. I wouldn't want to do this crossing in a rubber raft under rougher conditions. We were prepared to cancel the trip if we found rough water.
The beach on both sides is barnacled cobble. We had to be careful launching our rubber rafts. I brought beach shoes for launching and dry shoes and socks for hiking once we were across. As we approached the island there was a notable northwest current close to shore. You need to account for this with your bearings (note, the current would be different after low tide).
Once ashore we pulled out the trail maps and compared them to the NW Trails maps on our Garmins. Neither are particularly accurate. We saved our tracks to send in to NW Trails but we didn't map the whole island.
We first hiked up to the Chief Sealth's Birthplace cache. It was a pretty easy hike with most of the elevation gain right at the start. Once there we debated back tracking along the mapped trails and looping around to the Blake Island Summit cache. Instead we elected to bushwack about 600 feet to the other trail. This turned out to be a good idea. The forest was fairly open and a small game trail was easy to follow. That saved us a mile or two of extra hiking. The second cache was as easy to find as the first.
Then we hiked down to the NW corner of the island and followed the island loop trail east to Tillicum Village. We arrived at the village just before an Argosy Cruise boat arrived and disgorged a load of tourists. That was interesting.
From their we followed the shoreline trail around the east side of the island and back to the South End Campground and our boats. Total trail hike was about 7 miles. Total water crossings were about 2.5 miles.
We used the tide tables for Point Vashon. We found the actual tide change at our crossing was about a 1/2 hour behind the Point Vashon tide table. As we approached the Southworth side on the way back there was a notable current running to the SE, bringing us to shore a little east of where we were aiming for.
All in all I'd say this crossing is very doable in a modest craft provided that you are an experienced boater and you have planned ahead for favorable tides and weather. Any time you're crossing part of Puget Sound in a small craft there is inherent danger. Keep that in mind and you'll be fine.