There are lots of places to stay along I-4. If you want to got to SeaWorld, go to their website, they sometimes have very good deals on ticket/hotel packages. They also tell you which hotels give you a free quick que (like an express pass) if you stay there. We stayed at the Fairfield at Seaworld, and it was a 5 minute drive to the park.
I would rent a car. I don't think there is very close hotels to Legoland, and the drive up and down 4 isn't bad, especially if you do not mind highway. I drove from the Fairfield to BGT and back without issue. It was about a 2hr round trip drive. We passed the exit for Legoland about 1/2 hour or so from Seaworld on our way to and from BGT.
Good luck! There is so much to do in that part of Florida.
I agree with Lynne G about looking for some package deals on Sea World's website. The packages with the official SW hotels offer some nice extras and they are running some deals now like buy 4 nights get 1 night free, kids free tickets, free all day dining, etc. Definitely worth looking into. I also agree with the suggestion to rent a car if you plan on venturing out to Legoland. Have a fun trip