Best exercises for swim strength?...

Check around at your local health clubs to see if any have a Vasa trainer, then see if they offer one week trial memberships.

+3. I don't have the hard data to show whether it really improved my swim speed at all, but subjectively, I feel noticeably stronger in the water, at least for short distances, and definitely during sprints, when I'm in good shape on those 3 movements. Particularly the pullups (lat pull downs.)

Unfortunately, no substitute for the water though - it's amazing how subtle the correct application of the pull force is. I'm still working on making my pull while breathing left equal to that of my breathing right, and it's still devilishly hard to equalize - I get a good EVF while breathing right, but right now, no matter how hard I try, it's not as smooth or powerful when breathing left. 

I prefer straight-arm-pull downs instead of lat pulldowns.   Generally closer to the swim stroke motion and won't "inflate" your lats.  The straight arm pulldowns also works a broader range of back muscles.

Start with your arms straight out on a pull down machine and certainly no higher than level - even a little lower than level would be good.  Keeping straight arms, knees slightly bent and core engaged (hips slighlty forward and aligned), rotate at the shoulder downwards, then bring it back to level in a controlled manner.

Use weights such that it is easy to repeat 50x without a burn.  Do 3 sets, preferably within a circuit of other exercises.