If you are concerned with seasickness, then you will find the calmest water on the west side along Seven Mile Beach.
Most of the boats that Red Sail use along the west side are styled like pontoon boats. Those do quite well for minimizing motion in the light chop along that side.
The boats which Red Sail sends out to the North Wall are styled differently. The hull is more suitable for the rougher seas which can occasionally be found along the North Wall. While those boats do cut through the waves quite well, they rock and roll a bit more when sitting at the mooring in chop. Those boats are much like those we use on the East End where I work for Tortuga Divers, a Red Sail operation.
The good news is that, on average, summer brings calmer seas.
Hope that helps. Where are you staying?