How to survive a fall:Teleport - The teleport spell/clickies/scrolls fail on a 1 which will put you high above the marketplace, likely leading to death from fall damage if you do not have featherfall equipped or successfully teleport again before hitting the ground
Jumping off the front of the guild ship into the market or via a failed teleport will deliver roughly 85 points of damage* to a toon that has at least some tumble skill**, so it's far from an instakill for most toons using a teleport spell or even scroll. Other than "bottomless pit" type falls that only instakill, that seems to be the terminal velocity for the game: if you have any tumble skill and 90 HP, you're likely to survive any fall not into lava or other dotting substance.
*Your toon can bounce from a fall and take x2 damage, so 170+ HP is advised if you want to test it in HC. You can also aim for the edge of the bridge in the market and take [almost] no damage.
**I've not tested this fall with toons not having at least 1 tumble skill.