Grumpy is right. When I first started fishing for skipjack, I didn't have a sinking line, so I just used a sinking leader attached to a floating line. It's a little awkward to cast, but it worked.
The 6 ips sink tip ought to work fine. Usually you don't have to get down very deep for the skippies - just 4 or 5 feet is plenty. Remember in most places the canal is only 10-12 feet deep anyway on average. On tough days you might have to count it down some.
bd