Indonesia's fast boats are short hops rather than voyages, and the six crossings bookable here show the pattern: Bali to Nusa Penida in half an hour, Bali to Lombok and to the Gili Islands in an hour and a half to two. These are short island crossings measured in minutes, not the overnight inter-island ferries the archipelago is also known for.
Two things about boarding are worth knowing before you arrive. Be at the pier at least thirty minutes early — on the busy routes boarding is first come, first served, so a ticket alone does not guarantee a seat. And large bags are generally stored separately on the boat rather than kept with you; there is no strict weight limit, but expect to be parted from a big rucksack for the crossing.
The routes sell through the same aggregators that cover the rest of the region, which is the practical way to compare a fast boat against a slower ferry on the same pair. The departure point matters: Sanur and Padang Bai are two different harbours on Bali, serving different islands. Most routes can be booked as a return, sometimes at a small discount — though flexibility on the return date is usually worth more than the discount.