All- thank you for your replies.
Jim, Im quite certain I bought my Canon 300 (rebel) in 2003 or 2004... I was taking pics of my kids at sports events and the like with it (checking file folders). It is 70-300mm EF w/ image stabilization, f4-5.6 (gold).
There's a myriad of lenses to choose from; the one package i was looking at is thru Costco (red / L 24-105 and the 'same telefoto' w/ IS) along w/ a camera bag. The local big camera retailer here was matching it, but was offering the 'non IS' tele.. (surprise?)....
Never thought that a good size spend on cameras would be similar to buying a car!
The 70-300/4-5.6 IS USM was released in the latter half of 2005, 2 years after the 300D was released. 6 months prior to the lenses introduction, the 300D was discontinued. Are you sure it's not a 75-300 you've currently got?
What you've already got is largely irrelevant - if you're after quality telephoto reach for a modest sum, I'd recommend against using a 6D with a budget xx-300 zoom. The results will be underwhelming as the lens will be soft, and to make matters even worse, you'll be cropping loads.
For lake based water sports, you either need a motorboat to get up close (still around the 70-200 range), or serious reach.
If a motorboat is practical (are the coaches/regatta organisers using one already?), you'll get the best shots. IS is essential out on the water, even at high shutter speeds - if you can stretch to it, get a 70-200/4 IS, but if you can really get close without giving off too much wash, the 24-105 might just do it. Moving with the boats shouldn't tax the AF too much, so a 6D is good.
If a motorboat is impossible, quality reach is everything. The already mentioned 70-300L is worth every penny. Combine that with a good crop camera (7D or 70D) for an effective 480mm field of view, and it might just about be telephoto enough and sharp enough to give you something meaningful to crop into in post.