At most water taxi terminals, there will be several operators doing business at a variety of prices. Some are offering guided explanations, and others are simply more expensive or more direct (less stops). For example, we planned to take a boat from the Central Pier Station to Tha Chang (Grand Palace), and when we arrived at the terminal and started telling people where we wanted to go, we were immediately offered a fare from one company of 150 baht (5 dollars). But we dug a little deeper and found that a company called Chao Phraya Express Boat is the one the locals use, and it cost us only 15 baht (50 cents) for a one-way trip.
Five dollars isn't exactly breaking the bank, so it's easy to see how tourists get sucked into taking other boats when they aren't aware of the options. If it is the case that you want a guide and some extra perks (like bottle water), by all means use another company (just make sure you inquire upfront about what you get for the price). If you just want a ride, Chao Phraya is your cheapest and most frequent option. Unless you really plan to ride the boat all day and the entire length of the route multiple times, the 150 baht ($5) all day pass is way more expensive than you'll need. As mentioned, our trip from Central Pier to Tha Chang (30-minute ride) was 30 baht ($1) round trip.
Look for the sign in the cover photo at the pier to make sure you're boarding the right boat. There's a fare table on it that will ensure you don't overpay.
[Photo: Will McGough]