Can I request that you avoid top-posting any reply over the original message text. Rather than use email to reply, generally it is best to use the link in the footer of any email from the forum. The link will take you directly to the topic where you have the benefit of seeing any posts written since the email to which you are replying.
In the course of a developing topic top-posting a reply generates a vast amount duplicate text which makes it very difficult to find information later when searching old posts (and in your case it also adds masses of irrelevant data from your company's email servers).
To your question...
A SeaHawk draws some 18" with the keel raised, so you just need to add that to the distance between the slipway and the base of the keel. In theory that should provide just enough to float your boat off the trailer.
However, that theory assumes that the slipway is horizontal which, of course, they never are, except on ones in tidal waters that extend to the low water point.
Exactly how much more depth than the minimum that you will need will depend on the angle of the slipway as the aft may begin to float well before the bows and whether your trailer has rollers that will allow the boat to slide easily when half afloat.
Also bear in mind that launching it is one thing, recovering it may be another, especially if the slipway is steep and you really do have to haul your boat onto the trailer. Having well oiled rollers on the trailer definitely helps.