I did Sharkfin in a long day at the end of June a few years ago. It is doable if you go very light and leave the car at dark o'clock. When we did it, there was snow in Boston Basin down to the campsite, which made for very easy and fast travel up to the base of the tower. We made it back to the car around 5:30pm.
I haven't been in Boston Basin this year, but I'll bet the generally lighter snowpack this year may slow you down a bit.
It was a very enjoyable day, more so since we didn't have to schlep bivy gear. The fact that we didn't bivy also eliminated permit hassles.
[This message has been edited by Steve Kawasaki (edited 07-18-2001).]