HYANNIS — It is the weekend before the big July Fourth holiday jamboree and guests at the Cape Codder Resort and Spa in Hyannis are already making like the party is on: messing around in the wave pool with the kids (or without), drinking and eating on the outdoor deck, or just sitting on the adjacent shubbery-edged lawn and listening to a local band play hazy, lazy Eagles covers.

Just feet away from this slice of idyllic Cape Cod summer life, tucked away behind recently built walls, workmen are busy in relative secret, laboring away on what will be the Codder’s Indoor Water Park. The infrastructure is all there: a lighthouse in miniature takes center stage, a whale awaits a coat of paint, a ship peeps out from under some tarps, and two big body slide tubes snake through, around, and over these maritime icons. It was so close to being completed before the Fourth, but makes its debut this month with plenty of summer vacation time left.

The Indoor Water Park extends the Codder’s much-loved wave pool, which was built in 2000, shortly after Catania Hospitality bought the property, which was formerly a Sheraton Four Points hotel. The wave pool’s 2-foot waves, 50- and 80-foot water slides, and 24-person whirlpool are now joined by two 160-foot slides, and an extra 20,000 square feet for a total of 30,000 square feet of splashing fun.

“There are a lot of water parks and they keep getting bigger and bigger,’’ says Bill Catania, president of Catania Hospitality Group. “At 10,000 square feet, the wave pool was pretty small in comparison. It limited us in that market. You have to keep up with the times,’’ he said of the decision to expand.

Separated by an existing wall, the wave pool, which remained open during construction, is united with the new space by ground level entrances and, cleverly, via the wave pool’s ship’s hull, which was extended into a whole ship that sits between the two areas.

The water park adds a new café, the two-tier Gazebo, serving snacks and beverages, including a full bar. There’s also a separate water play area for toddlers and younger children, complete with a smaller slide, a mini pirate ship with water canons, and a dumping bucket. “It’s fenced in for safety. It’s a mini-park within the water park,’’ adds Catania.

Accessed by steps inside the lighthouse, the thrill ride through water slides is a focus for bigger folks. For a more chill ride, however, there’s the option of grabbing a tube and floating through the park on the 300-foot-long Lazy River, passing waterfalls and around the ship. Chill, that is, until automated special effects turn the river stormy.

“You’ll be floating around and then thunder and lightning starts, and it rains. The Lazy River turns into an action ride,’’ Catania says. “The humpback whale also spouts water every once in a while. And at night, LED lighting transforms the place. It looks really cool.’’

But Catania went beyond the fun factor in creating the new water park: In addition to filtration and chlorine, it has the same high intensity ultra-violet light system used in the indoor wave pool and Rainbow Falls heated outdoor pool. Though not required by state or local laws, the system is an added tool to destroy bacteria and bugs in the water.

The water park, which replaces a volleyball court and a grassy play area, has a retractable, clear roof, which makes the park weatherproof and therefore year-round, a very important factor in attracting off-season visitors. “In the cold months, it will be a great addition, especially for people who don’t have the time or don’t want the expense of going somewhere hot in the winter. In summer, it’s outdoors; in winter, it’s indoor.’’

Hotel guests have unlimited free entry to the water park, of course, but a limited amount of day passes are for sale, allowing non-hotel guests to use the park, too. For safety, guests must be at least 42 inches tall to use the slides.

In addition to the water park, which broke ground last November, the Cape Codder underwent considerable interior renovations over the winter. Some rooms adjacent to the water park were converted into larger family suites with a kitchenette. One surprise feature in the room is the beautiful aqua blue table, which turns into the cutest Murphy bed: “We wanted an area for dining and playing, but also with the additional bed, the room has the ability to take families of different sizes. It can comfortably sleep six, and we can add a cot.’’

Draw back the curtains and the room’s windows overlook the water park. “It’s right there,’’ says Catania. “People can just jump right in — almost.’’

CAPE CODDER RESORT AND SPASummer rates from $359, winter from $169. Water Park: 9 a.m.-9 p.m., daily. Day passes: $40, $30 for children under slide height. 1225 Iyannough Road, Hyannis. 866-976-8454. www.capecodderresort.com

Linda Clarke can be reached at soundz@me.com.