The Marriott on Kauai is in Lihue. I've never been there - drove past a couple times - but the beach in that area is often rocky - more of a sunning beach than for swimming. One of the other resorts nearby has a sandy beach pool because of it. There's only one shore dive near there - Ahukini Landing. Don't touch the unexploded ordinance.
The better diving is off the Poipu area - 15-20 mins. south. It's likely why both Seasport and Fathom Five are based there - also 5mins. to the boat harbor. The signature boat dive - Sheraton Caverns - is just offshore there. And Koloa Landing, the better shore dive - is nearby also. There's big turtles living at Sheraton Caverns but we didn't see a lot of sharks - just a couple at Fast Lanes.
The Marriott in Waikoloa (Big Island) is about 20miles north of Kailua/Kona. Most of the shore diving is south of Kona - although there are a couple of sites north of Waikoloa. A lot of that coastline - except for the beaches - is jagged lava so entries are difficult. Almost all the boat operators leave from Honokohau (sp) harbor - just north of Kona. So you'd have to drive down daily - not far but it gets old.
For big pelagic diving almost nothing beats the Manta Night Dive. Pelagic Magic might be another obvious choice. Jacks Diving Locker has a video on their website about it. My buddy was there last year and in addition to diving with 12 Mantas, they twice dove with big pods of Spinner Dolphins south of Kona at two boat dive locations - once they estimated over 100.
I did find an operator who keeps a boat just north of Waikoloa once on google.
IDK where the other Marriott is either but if it's south of Kailua/Kona - likely near Keahoeu (sp) - that would be closer for most shore diving. There's about 5-6 nearby sites plus Place of Refuge a short drive south. There's one big hotel in that area (maybe it?) where they turn on the over water lights at night to bring the Manta's in for viewing. But the dive is done elsewhere. And I think the entry there is too rough over the lava to get out to it - at least from what I've read.
The most accessible diving is from Wailea on Maui. There's also more sandy beach entries nearby - Kona often tends to be lava up to the waterline. The other thing about Maui is that all the diving is compressed along the west coast - within an hour drive you're all the way up to Honolua - about the farthest north site. Wailea has several good shore sites. The Marriott is between two - Ulua Beach and Wailea Beach. Makena Landing/5 Graves is a few mins. drive south. There's others also.
The Marriott in Honolulu is one of the 20-30? towering hotels next to each other on Waikiki Beach. Diving is elsewhere on the island except for one dive at the end of Ala Moana park. We went to the Waikiki Aquarium instead - it's small. If you're ever in the area, the most amazing thing is how so many people fit on Waikiki Beach.
Shore dive options on all the islands:
Scuba Shore Diving Region: Hawaiian Islands