We did a daytrip on the Kalalau trail when we were vacationing at the Princeville on Kauai. What an awesome hike, just amazing views. We were there in February last year, and rain was a major issue for about an hour on that trip, I thought my wife was going to slide all the way to the ocean at one point.

Since we were only doing a day hike, we brought all our water with us, I think my son and wife had 2 one-liter Nalgene bottles each, and I carried that plus 3 quart bottles of Gatorade. (Me thinking I'm super-dad, I humped all the extra gear, drinks, bivy bags, lunch, ... what a deal that was, am I a dope or what?)

Anyway, if we were going to overnight, we would have needed a permit, which we didn't have. To do that, water would be an issue, and I was told NOT to drink the water, due to the lepto bacteria that you mentioned. I was told that you MUST boil the water out there, the filters can't get that bacteria apparently. Even though we had about 9 liters of liquid between the three of us, it was virtually all gone by the time we came back from the day hike, you do get a workout going up and down that trail along the coast.

I am jealous. You are in for a real treat, that's one of the most beautiful trails I have ever hiked. It can be challenging, especially if you get the usual afternoon rain. We came back off the trail glad to be climbing into a rental Trailblazer, instead of a car we own. I think the seats were covered with that red dirt which was all over our boots and pants.

Have a great time!