Geckot, there are many waterfalls in Lombok. Which one are you thinking of visiting? I haven't seen a waterfall that wasn't beautiful yet
The one's I've visited in the last couple of years is at Senaru and Ganga.
Senaru has 2 waterfalls, you walk down about 100 stairs to the first one and it is quite nice, but the second one is about 20-30 mins walk in through the jungle and it is really lovely. Yes you can swim (but it is COLD). Depending on how fast the water is running depends on how close you can get to the cascade. If a lot of water it is too strong and you can't get close, it takes your breath away.
Ganga you walk up through rice paddies and then climb down through rocks to get to the waterfall. Maybe 30-40 mins?
I recommend for both that you hire a local guide. Not expensive, and they will show you the best way into the waterfalls.