Most Phuket island tours run on either a speedboat or a larger multi-deck boat — the choice affects comfort, speed and stop count more than most people expect going in.
Speedboats
Faster, meaning more stops fit into a single day, but rougher in choppier conditions and with less shade and shelter — better suited to calmer-water routes or travellers not prone to seasickness.
Big boats (ferries and larger tour vessels)
Slower and covering fewer stops per day, but considerably steadier in rougher seas, with proper seating, shade and sometimes on-board facilities — often the better choice for families, older travellers, or anyone genuinely worried about seasickness.
Which matters more: season or boat type?
Green-season sea conditions (roughly May–October) affect comfort more than the boat type alone — a big boat in genuinely rough conditions can still be uncomfortable, and a speedboat on a calm day is often fine even for sensitive travellers.
A reasonable approach
If you get seasick easily or are travelling with young kids or older relatives, default to a big boat tour and take motion sickness precautions regardless of boat type during green season.