I would also recommend: Lightning. During a heavy storm, can strike down and stun or give some level of wounds to a creature (depending how much conductive metal he's wearing); lacking a continuous stream (say, ending one tile radius, despite whatever constructions are there) would help reduce any exploitation. Can also set trees on fire or burn the ground.
Swarm creatures. Swarm creatures are multi-tile creatures that have low health per individual, but can constitute significant damage as a group but have weak health. Unlike carp, where one is deadly, they can have other effects: Jellyfish warms can stun and potentially drown a creature (and turtles'd eat the swarms right up), locusts can attempt to consume crops and dwarven clothing, mosquito swarms can tire creatures and exert them rather swiftly. Piranha swarms, well, do we really need an explanation?
Tornados. If we're going to have hail and hurricanes, might as well have a massive wind column come down and move across the map, destroying whatever is in its way.
Seeing as we were going on dangerous terrain: Quicksand. Depending on a dwarf's agility (or perhaps tracking) skill can preempt the fun way of finding these, otherwise what appears to be normal terrain becomes a horrifying experience.
New uses of ropes: Climbing ropes can be used to help dwarves in the water climb back into a higher z-level than a slope. Might also be a nice siege device, allowing enemies to scale walls; this is something that can be stored in a backpack.
New use of thread: Nets. Right now, dwarves just do it the dwarvenly way and grab fish out of the water (vermin fish, at that!). Having a net might increase their net gain, or be a requirement to just get the job done. Can also use spears for the part (not as dwarvenly with dwarve hands), or less excitingly, one wood or metal or stone + thread == fishing pole.
I'm not sure how I feel about life vests. It's bad as it is that we can hardly control what they wear--why bother including an item they won't wear if they don't have an open slot for it?