Guild Wars 2

Yes, you must have your stacking weapon equipped at all times

You also can’t swap it to mainhand or offhand, and you can’t replace it with another wep of the same sigil.. which I think is something they should fix.

this is fixed now. you must now keep the stacker equipped at all times to get the benefit. even when under water because technically you are un-equipping your land weapons for your water weapon.

Have a sup bloodlust on my Ele’s staff
I caved and put one on my Ele’s Trident, to not loose stacks when going underwater.

What did work;
As for changing(unequipping) weapons, when underwater i can unequip the staff and not loose stacks. When on land i can unequip the trident and not loose stacks.