Mohegan Sun has upped the projected cost of its Palmer gambling resort to nearly $1 billion with the addition of an indoor-outdoor water park and a second hotel, making the rural project the largest in the competition for the sole Western Massachusetts casino license.

The project will probably include indoor surfing, a “lazy river” feature, and an extensive series of outdoor zip lines, said the developer.

The new $130 million to $150 million resort component, inspired by the Pump House water park at the Jay Peak resort in Vermont, will be developed by Boston-based Finard Properties, which has also joined with Mohegan Sun to build a retail complex at the development, said Todd Finard, chief executive officer of Finard Properties.

“People tend to use this kind of facility for vacation,” Finard said in an interview, speaking of the water park. “People come and stay for upwards of a week in order to enjoy the entire facility.”

By adding a recreational element designed to draw families, Mohegan Sun is looking to expand its potential customer pool and give out-of-state visitors another reason for extended stays at the resort, said Mitchell Etess, chief executive of the Mohegan Tribal Gaming Authority.

“It’s a complementary amenity, and it’s a draw unto itself,” Etess said in an interview. “And we’re creating that real destination resort that people want to come to and want to bring their money from out of state.”

Mohegan Sun is one of three prominent gambling companies in the hunt for casino development rights in Western Massachusetts. Mohegan is competing with MGM Resorts, which has proposed a casino and entertainment complex in downtown Springfield, and Hard Rock International, which has planned a casino and hotel resort on the Big E fairgrounds in West Springfield.

A fourth competitor, Penn National Gaming, had proposed a resort in Springfield, but was eliminated after an intense competition within the city. Penn National officials said they are no longer interested in developing a casino in Western Massachusetts and are exploring options in other regions of the state.

The state gambling commission, which controls the state’s three resort casino licenses, is expected to choose the winning Western Massachusetts project in early 2014.

Plans for the water park will include a “tremendous indoor component” and “an outdoor component that in some ways is limitless,” Finard said. “When we start to set the final plans for zip lines and horseback trails and some of the other outdoor elements that go along with this, it’s hard to really quantify.

“A guest will come during the winter perhaps for snowshoeing outdoors and indoor kayaking and indoor surfing,” said Finard. “During the summer, you’ll have zip lines to take you all around the property. A guest experience on zip lines could last hours. You could send them tower to tower and provide the kind of entertainment that we haven’t seen in this part of the country.”

The entertainment project, he said, “is massive.”

Plans call for the second hotel to be connected to the water park and detached from the main casino hotel. The second hotel would have about 250 rooms, Mohegan Sun officials said.

Clyde Barrow, a casino expert at the University of Massachusetts Dartmouth, said family-friendly amenities have a mixed record at gambling resorts. “It didn’t work when Las Vegas tried it in the 1990s,” he said. “On the other hand, out west, especially with the tribal casinos in rural areas, you’re seeing more of this. You’re trying to expand the customer base, and also the thinking is that while dad is playing blackjack, mom can take the kids to the water park.”

With the addition of the new components, the Mohegan Sun proposal is close to the projected development cost of each of the three resort casino projects competing for the Greater Boston license, led by a Wynn Resorts project in Everett, at about $1.2 billion or slightly more.

Greater Boston is believed to be a more lucrative gambling market than Western Massachusetts, and Barrow said he remains skeptical that the potential return in the west is big enough to justify the large scale of the three competing proposals.