When a boat sinks do you get sucked down with it?

Unless you are attached to the vessel, you will not get sucked down by any suction caused by the vessel going down. You may temporarily go under the water but only for a second or so. Different people float differently. Those with low body fat will not float as easy as someone with a higher fat content. If you have a life jacket on, (as you should), you will float around.

I have 20 years as a Boatswains Mate in the US Coast Guard and have never heard of a person being "sucked down" by a sinking vessel.