YA Fantasy novel where kids learn to open portals between worlds.

[Kody]

Probably written in the early 2000s, major plot points I can remember:
-If the person who opens the portal dies, everyone who goes through that gains the power to control all of the cells in there body. Essentially they can shapeshift. This was something the bad guys did and horrified the protagonists.
-Large portals allow old wooden ships and crew to pass through them.
-Other magic is used, at one point a spell is used to remove all the oxygen from the water below a ship in order to kill all of the sea monsters attacking from below.
Thanks for the help!

[Lobstergirl, au gratin]

Kody, are you still looking for this or did you find it?