Thailand's ferries are how the islands connect, but almost none of that inventory starts where a traveller starts. This page lists four bookable crossings, all on the Gulf side, and each one is the sea leg of a longer journey rather than a sailing from Bangkok itself: you reach the coast first, then the boat.
That is the thing worth understanding before comparing options. The mainland gateways are the places to search from — Donsak for Koh Samui, with departures through the day; Chumphon and Surat Thani for Koh Tao, Koh Samui and Koh Phangan; Krabi for the Andaman-side islands. A search that starts inland returns a package, and the boat inside it is chosen for you.
Boat type decides the crossing more than the operator does. Speedboats and catamarans take one to two hours on most routes and are rougher in swell; car ferries are slower and steadier, which matters in the rainy season and to anyone prone to seasickness. At the larger piers, Donsak among them, a voucher may still need exchanging for a paper ticket at the counter before you sail.