Perhaps I missed it but I didnt see rolling hitch on this thread: surely part of anbody's core set of knots? certainly one I use routinely at sea, where you often rollling hitch a thinner line round a thicker one to have give a purchase that slides when pulled one way but grips the other way, although I also use prussiks for this (a heavily used rolling hitch will often twist itself into a tangle, a prussik not so much).
Less well known but incredibly useful is the tugman's hitch: can be tied and untied under tension, can be tied in the bight, very little slippage, will never come accidentally undone, easy to untie. But very bulky! The perfect knot for tying to a bollard or similar.
Less well known still is the buntline hitch: more compact and more secure than round turn and two half hitches, although can be impossible to untie when it's very tight. A lot of people without thinking about it will use a half turn and two half hitches to secure a line to a post or spar, but I never use that at sea, using either a round turn and two half hitches (can be tied under tension, but prone to slippage and accidental release, and a bit bulky) or buntline hitch. An anchor hitch is a simple variation of a round turn and two half hitches giving greater security but to my mind doesn't do much that you can't do with a bowline or buntline hitch
Essential knots at sea are
1) Bowline (forget rabbits, holes and trees, learn the one-handed technique, much the fastest way when you have two hands)
2) Figure of eight stopper knot
3) Clove hitch (notwithstanding earlier comments a great knot: can be tied in the bight, you can pre-tie one ready to drop over a bollard, and you can make a simple quick-release version if you tie it in two stages by using the bight for the second stage)
4) Rolling hitch (or prussik---but you need a double fishermans or similar to prepare the prussik)
5) Round-turn and two half hitches
6) Anchor hitch or buntline hitch
I know mountaineers like the Alpine butterfly, it's certainly a nice knot; but the easy way of tying it doesn't allow you to tie it round something else; and it can be hard to untie, so I don't use it much. At sea, ease of untying can be as simportant as ease of tying!
Finally you need a method of joining two ropes: the recommended method at sea is a double sheet bend but in practise a lot of people will just use two bowlines. If you do use the two-bowline method you can reduce chafe by reef-knotting the lines first---but *never* use a reef knot alone to join ropes as it capsizes and comes apart if the tension on the ends is unequal. The only place I use a reef knot at sea is on reef lines.
If you want to take your knots to the next level then learn the carrick bend: a lovely way in its own right to join two ropes, but also takes you 90% of the way to a diamond knot, one of the few knots that work in super-slippy stuff like dyneema (and used for making your own soft shackles), and opens up the whole world of turks heads and similar. But a more practically useful skill is splicing, especially for dyneema, which I like but which I never even try to knot (except diamond knots), splices only, generally locking splices ...
