Sea World is a great place to visit. I want to start by saying that, and if you have your heart set on it, then you have your heart set on it. You trip will not be complete without it.
O the other hand, DIsney has attempted aggressively to keep you onsite. If you are already looking at getting 6day WDW tickets - then making those day tickets forthe whole family is likely to cost less than $15 - two extra days for everyone for less than the price of amovie!
Conversely, a trip to Sea World will cost you over $200. More if you have to get transportation! I think Mears is $14 per person, a rental car is at least $200, if you get a good deal.
To put it simply, a WDW vacation is an active one, at least if you plan on actually going on most of the attractions. Part of the thrill for our family is making it active.
Our preference is to take it easy for two days rather than do nothing for an entire day.
What I suggest is tentatively sticking to six day tickets, but consider upgrading to seven or eight day tickets later in your trip. (Do this AFTER your first day, it will cost slightly less than doing it when you arrive.) It only takes a few minutes to upgrade your tickets. (The difference between six day and eight day tickets is minimal, so it's also no big loss if you just buy eight day tickets up front and don't use them two days. ) You never know if someone is going to get sick, or twist an ankle or something that will force a day of rest. Personally, once I have eight day tickets, I sort of feel compelled to use all eight days, but maybe you won't feel that way.
The other two things I STRONGLY suggest for a newbie is that you start going for daily walks well before you arrive. I've even been to WDW with a runner who couldn't keep up all day because he wasn't used to WALKING all day! Second, at the end of each day, no matter how tired you are - soak your leggs in warm water. This could be a the hot tub at your resort or the bathtub in your room. If you are squeamish about hot tubs, just soak the lower half of your leg and massage your feet right on a water jet. Taking a Tylenol before you go to bed doesn't hurt either. A few minutes will do wonders for your legs!