I didnt start offering water until my CMHN suggested that i should (about 8mths), not because BF babies particually need it, but its a good habit to get them into as when you have weaned (when ever that may be) they will need to be able to take fluids from a cup.

I tried many many different cups (sippy and straw variety) and the one that she prefers the most is the 'take and toss' brand, i dont throw them away after one use, i have heard tat they are top shelf dishwasher safe, but i just wash with hot soapy water and then rinse...
It wasnt until just reently that she actually consumes much, it was more sip and swallow once and then sip and spit the rest of the time - it was a game for her, but it was there if she needed it and if not she got the message that it was there and by the time that she needed it (now) she knows what to do...kwim

HTH