If you’re ruling out Universal and Sea World, you’re probably best off adding one day to your ticket. Even with the best of intentions, once you’re there, it’s going to be really hard to resist the desire to get into the parks. You can always go to a park, take a break during the day, and then go back to the park in the evening.
If you don’t have a car, you’re probably going to pay somewhere around $40 or more for a round trip between WDW and Sea World or Universal, plus the tickets. For that kind of money, you can add a day and upgrade to park hopper or add the waterparks option. Maybe both.
Edit: The extra day for a non hopping ticket will only cost $3 per adult.
Six days of hopper would add $60 per adult.
Six days non hopping plus waterparks would add $60 per ticket.
If it were me, I’d add the extra day and park hopping. I refuse to be limited to only one park each day.
Here’s a helpful chart of all prices and all options: