Your understanding under the "Lost OB" section is correct. (Although I should point out that "lost" is different than "OB", even though the same rule applies when dealing with either.)
The "Water" section is essentially correct, with some exceptions/additions:
1) Technically the penalty for dropping a ball after it has gone into a hazard is not for a "lost ball" - after all, you might actually find your ball in the hazard (if it's near the waterline, or in a dry part of the hazard) but still choose not to play it from there. Technically the penalty is just a "one stroke penalty" for dropping a ball after one has gone into a hazard (just as it's a "one stroke penalty" for a ball lost or OB). The term "lost ball" is reserved for balls lost that are known to be *not* in a hazard, since a different rule applies in that case (essentially, your description in your "Lost OB" section would apply there.)
2) With a water hazard, you have three options:
a. Proceed under the stroke and distance provision of Rule 27-1 by playing a ball as nearly as possible at the spot from which the original ball was last played (see Rule 20-5). You say this is what you've been doing, which is fine, but as you surmise you're usually better off taking a drop based on where it crossed the border of the hazard, which is the second option:
b. Drop a ball behind the water hazard, keeping the point at which the original ball last crossed the margin of the water hazard directly between the hole and the spot on which the ball is dropped, with no limit to how far behind the water hazard the ball may be dropped. This, or a. above, are the only 2 options you have if the hazard is marked with YELLOW stakes. Note that you are allowed to drop anywhere, as far back as you want that is still in bounds, on a line back from the flag through the point at which it last crossed the hazard - not just at that point. (Note that by definition you cannot drop closer to the hole with this option.)
c. As additional options available only if the ball last crossed the margin of a lateral water hazard (which is denoted with RED stakes), drop a ball outside the water hazard within two club-lengths of and not nearer the hole than (i) the point where the original ball last crossed the margin of the water hazard or (ii) a point on the opposite margin of the water hazard equidistant from the hole. (In practice, most people who drop from lateral hazards use option (i), but option (ii) can sometimes be advantageous.) So with this option, you can still drop on a line back from where the ball last crossed the hazard, but you are also allowed to drop to the side, up to 2 club lengths away from the point on the hazard line that the ball last crossed, as long as it is no closer to the hole.
And of course, you could play a ball that's in a hazard as it lies with no penalty - you just can't ground your club.
Hope that's clear. You can see the actual rules (which I've copied and pasted much of) here:
http://www.usga.org/Rule-Books/Rules-of-Golf/ is a good resource for you if you have other rules questions.
Edited by sacm3bill - 2/25/13 at 4:47pm