The only way to get a watch is to wait for it to become available. No one outside of a very few at Apple know for sure what watches will be available when. The only way to insure you get one at the earliest date is to either pay a premium to someone on eBay or Craig's list, or get into the "virtual" line at Apple and wait your turn. As melman said you can always cancel if you find another route to get it sooner, but if you don't reserve your spot in line, you have no assurances any other plan will work. Your CC is not charged until Apple starts packaging your watch, and in that case it arrives at your door the next business day.
Just checked on the US Apple Store iPhone app and the 42mm SG purchased as of now is shipping 2-3 weeks. Some models are now 1-2 weeks, the only one I saw shipping longer than 3 weeks was the space black.
Yes. Now is the perfect time to wait for in store a availability. I anticipate in store sales next week due to the 2-3 week shipping estimates. If by tomorrow or Friday Apple improved shipping again by posting 1-2 weeks, then either buy it right there online or wait. I have a good feeling that first week of June is in store availability.