Approach from Boston Basin as described for all other routes on the mountain. I have something special to say about Sharkfin Col however. While approaching the climb we met a fellow with some knowledge that made dealing with this unpleasant obstacle much easier. (thanks again "Nelly"!)
The col is at the low point in the ridge between Forbidden and Boston Peaks. Selected Climbs in the Cascades recommends leaving the Quien Sabe Glacier at it's western edge (elevation 7,500 feet). Then, as normally described, you would climb some steep, loose rock (often rated 5.7), to enter a gully of loose 3rd class that brings you to the col.
But instead, we easily avoided technical terrain by climbing a hidden 35-40 degree snow couloir immediately on the right, not visible from the usual overview-of-Boston-Basin pictures. Climb this colouir for 200 feet to reach a notch in the ridge just 20-30 feet higher than Sharkfin Col. There is a decent rappel anchor here with fixed slings. We had a single 30-meter rope, and one rappel got us to a flat rock at the edge of the Boston Glacier.
Our climb was in August, and the snow couloir was full of snow, after what was I believe an average winter (2001/2002).
Once on the Boston Glacier, descend 200 feet to get around a protrusion of rock coming from the Forbidden-Boston ridge. Then, follow the glacier as crevasses allow to a broad snow entry onto the North Ridge.
A rare snow break from one of the longest rock ridges in the area.
Once on the North Ridge, you have a choice: either begin following the ridge with 4th class and low 5th class rock climbing...going all the way to the summit. This is what our party did, and we found it aesthetic and enjoyable. We did have a pitch pretty early on that we rated 5.6, and that was the hardest climbing of the trip.
Many folks instead descend from the ridge on the west side, possibly making a rappel or two, only to turn back up a 40 degree snowfield that brings you back onto the ridge 5-6 pitches from the summit. I really don't know why one would go this way when a pleasing ridge to the summit is right there begging to be climbed :-). Either way, this variation regains the ridge at an intrusion of permanent snow. On reaching that point from our rock variant, we had to put crampons on for a few hundred feet before regaining rock on the other side. Indeed, the rock becomes very, very solid after this snow intrusion.
From the summit, descend the East Ledges or the West Ridge as described elsewhere. We went down the West Ridge, but in hindsight, I think the East Ledges would have been quicker.
The climb has an outstanding position, with particularly compelling views of the Direct East Ridge, with snowy peaks of the Ptarmigan Traverse region peaking through dramatic notches in that ridge. I found the route very enjoyable because of the long length of the ridge...it seemed to go on forever, and we could really lose ourselves in the climbing.
We found the route doable in a long day, given good weather and visibility. We were aided by the easier version of crossing Sharkfin Col, by a comfort with long stretches of simul-climbing or soloing on the ridge, and one of our party had familiarity with the West Ridge for descent. We started at 2:30 am, and were back at the car by 5 or 6 pm. The chief obstacle to success was the recent massive avalanche that destroyed a portion of the Boston Basin trail. We lost the remains of the trail completely and bush-bashed up into the basin in the dark. Not fun, but perhaps an echo from the pioneer past, worthy to experience!
Bring a 30 meter rope, possibly a half-rope that you can double for the rock ridge. We only found one pitch that needed a fixed belay.
Bring crampons, ice ax, glacier gear like prussik cords and a picket.
Bring a light rack of nuts and cams for the rock climbing.