Depending on the venue and the day of the week you're there, you may be able to get in early or you may have to wait a bit. In general, the earlier you arrive for dinner, the better. In the case of Crazy Crab, it's likely you can get in without too much wait around 4:45-5 p.m., though the parking lot is full at that time (I drive by it often at that time). Others, like Skull Creek Boathouse, can have a waiting list already by 5 p.m. As mentioned, there are a number of very good restaurants that do take reservations, though you often have to use a reservation service like Ressie or Open Table.
You mention grilling at the resort. There are several located around the resort, so most buildings have one within a short walk if not literally right outside the building itself. They require charcoal, so you will have to provide your own. The Kroger at Shelter Cove is very convenient--you could walk there is you wish (10-12 minutes) or it's a short car or bike ride away. If you want fresh seafood to prepare, Barnacle Bill's is less than two miles down the main drag, though he tends to sell out early. Another few miles down, just before you cross over the two main bridges between HHI and Bluffton, is Hudson's Fresh Seafood (not the same as the restaurant), and they have a wider selection and usually don't run out of items. Both venues are excellent.
As for the pools, the main pool tends to fill up fairly quickly. Some people get there right at 9 a.m. to grab the chairs they want, but you can often find open spots as late as 10-10:30. After that, you have to put your name into a virtual queue where the CM will text you when chairs become available for the size of your party. The Beach House pool has no lifeguard, therefore, no queue, and even when all the deck chairs are taken, there is still plenty of seating by using the tables under the awnings. That pool area never closes due to capacity, plus there's an indoor area--Surfmen's Station--where you can also relax and get out of the heat. There is no slide there, but given it's right at the beach with a private gate, you are merely steps away from walking on the beach. The issue there is parking. Most days the Beach House parking lot is full by 10:30, so taking the shuttle is your best bet unless you feel like biking or walking over (1.5 miles one way) on the lovely bike paths. Around 3:30 or so, the parking lot begins to slowly empty and spots are available. The BH is open until 7 p.m., currently, but the shuttle only runs 9-5.
Have a great trip!