Swimming:
Player: I swim.
CK: Ok, you are swimming.
Unless they are overburdened or have some other reason for not being able to do it, it doesn't require a check. If they are doing some fancy swimming; like going upriver or dodging walrus or something, then you might call for rolls. In my game I had some swimming going on, and the barbarian did the all out physical exertion thing and could do extra things, like chase after a boat and overturn it. The others may have made checks when doing those things.
Note that fairly complete rules for swimming are in CKG, p. 145. Discussed are those unfamiliar with swimming, aquatic creatures, specific encumbrance effects, and drowning rules.
Escaping a landslide:
It's a wisdom check for surprise. Then it's a matter of moving fast enough to get out of the way. If you are caught in the zone of the slide, it's a dexterity check. Base the CL on the overall competence of the adventurers instead of the actual level of the challenge (if you intend it to be beatable). e.g. for 1st level characters, make it a CL 0 check. If you want to base it on reality, I have no advice

.
For examples of intelligent rocks out to get you, look to the module D0: Dwarven Glory - Looking Stones
Building a Boat:
For some value of boat. Considerations are time, materials, and expertise. Have you ever built a boat? Have you ever seen a boat? Are there trees, tools, etc? Again, CKG has information (p. 57) Time to build is in _months_. No throwing together a canoe overnight. Assuming materials, tools, and some expertise is present, I'd just wing it. No check, just time. Of course, once it's on the water I'd make a roll to see if it falls apart, maybe roll above a 10 to stay afloat, modified at +/- 1 per important aforementioned concern addressed in construction.
Can they sail it? Int or Wis taking into account their background.
Hunting, foraging:
These are class skills (Survival, Wilderness Survival, Nature Lore). If no one has them, they don't add level.
Sorry if the responses are simplistic. If you have more information maybe we can do better.