I agree with ReyeR, WAL without a strobe will have rather narrow application. Shallow dives with lots of sunlight, silhouette shot, big subject such as whale shark, whales would be something that you can use WAL without strobe. However there are plenty of other situations where you would use WAL and you do really need a strobe. Of cousre strobe is quite a bit more expensive than WAL but I think between the two, you will get more flexibility with a strobe first.
For WAL, I generally recommend Inon UWL-100 mainly for an option later to add a dome lens (so you can get about 140 degree view rather than 100 degree view with WAL only).