When you spend your hard earned money on a guided fishing trip, don't take anything for granted. I've hired good guides, poor guides, and just plain slugs for guides. Rule of Thumb: If you're looking for a guide on short notice, don't expect too much. If someone is available at the "drop of a hat", in most cases they are not worth the trouble to hire. Most good guides are booked well in advance. Use word of mouth, whenever possible and book the guide well in advance...sometimes a year in advance.
Nonetheless, especially when going somewhere for the first time, you owe it to yourself to be well prepared. The guide should have all the flies, lures, terminal tackle you need. If fly fishing, they should have replacement leaders and tippet material. But I never count on it.
It's always good to have certain items with you. There items are best carried in a "boat bag" that is waterproof and is of reasonable size. Reasonable size is 16" x 10" x 6". This is big enough to carry all the items you may need, but not too big to be cumbersome and unwieldy. Backpack type bags are OK, but should have several compartments to you're not constantly digging around looking for items.
Following video and accompanying list, will help you determine the essential and optional items to bring along to ensure you'll have everything you need, regardless of what the guide has in the boat.