You definitely want to take water, but respectfully, I suggest a refillable bottle. Plastic water bottles are the scourge of our planet, and largely unnecessary.
I have never been on a dive boat that did not provide drinking water. Nevertheless, I too always take a water bottle, just so I have it handy where I am sitting. The less you have to walk around the boat, the better. And if it's hot, a bottle of sports drink, too.