I'm trying to venture under the sea, but I've been told that my familiar (a mosquito) wouldn't be able to breathe! Do I have to leave him on the surface, or is there some way to kit him up for aquatic adventures?
When you first meet the Old Man, he should give you a little bitty bathysphere, which allows you take non-water breathing familiars underwater at the cost of -20 pounds. After retrieving the Old Man's boot, you have the option of receiving the das boot familiar equipment, which also allows non-water breathing familiars to adventure underwater with a reduced cost of -10 pounds.
Another way to adventure with your familiar underwater without a penalty to familiar weight is to use willyweed, which is an item that your penpal can send to you. This, however, is a limited-turn item effect unlike the familiar equipment.