Most often, it's not an issue. During the day, you're generally riding your bike while the boat makes its way to the next location. Overnight, you're docked at a pier.
There are a few exceptions: Sometimes, the boat is underway at night. However, river and canal-centric tours don't present the rocking element that the sea sometimes does.
On some river and canal tours, the boat is occasionally underway during the day, but there's little sensation of motion.
On some of the bike and boat tours along the coasts, you may be underway during parts of some days. Although you're on the sea, though, you're mostly sheltered by islands, so the large swells that tend to induce seasickness typically don't develop.
If you have concerns, please check with our tour advisor team, which can advise you on the flattest, unrockingest bike and boat tours.