11:57 AM PT -- Riley Strain's family just held a press conference, announcing they've enlisted the help of the United Cajun Navy -- a nonprofit that helps in this sort of effort -- while reiterating the need to get more resources on the case ... and also thanking Nashville PD for their work thus far.

Police went on to say that they've been inundated with tips and leads from various sources, but none have resulted in anything concrete in terms of locating Riley and there is no indication of foul play.

Riley Strain's family is 100% confident that their loved one is still alive somewhere -- and they're frustrated with cops' search efforts ... demanding extra boots on the ground.

We talked to the Strain family spokesperson, Chris Dingman, Tuesday and he told us straight up ... Riley's parents, simply don't believe their son is dead -- this despite most of the public starting to assume that's the case at this point.

Even though it's been well over a week since Riley first vanished in Nashville after bar hopping with his friends -- Dingman says the family is hopeful ... but more than that, they're irked that much of Nashville PD's investigating has been focused mostly on the river nearby.

There's a few reasons why Dingman says the Strains feel this way -- for one, he says the river just isn't that deep, and it doesn't appear to have been roaring on the night he disappeared. So, in their eyes, him falling in and being swept away doesn't seem plausible.

Dingman also tells us that from the family's POV, cops haven't turned over every possible stone in trying to track him down -- claiming there are businesses along the river bank with surveillance video that weren't approached until very recently ... so, slow going at best.

While the Strains appreciate the police's help and diligence thus far -- we're told they think it's time to call in the cavalry and get extra assistance in helping find Riley sooner rather than later.

Nashville PD has already said they're prepared to call in the feds if it proves necessary. Dingman says the Strains think that time has come ... and that the sweep for Riley needs to expand well beyond just the river search that's been going on these past few days.

Of course, Riley's bank card was found there -- which is probably why this is a focal point in the effort -- but Dingman says his mom and dad want other possibilities looked into.

He doesn't have an exact idea of where he may be or what happened to him, but based on eyewitness accounts that continue to crop up day after day ... it sounds like people might've possibly been with him during the last known moments of him being out and about.

The other odd thing that's not quite tracking for the Strains is that when Riley was walking by the police officer -- he came across as coherent and normal, but other surveillance videos captured him looking inebriated and out of it. So the family doesn't know what's what here.

The one thing they do know ... they want answers, and soon. The Strains have been in Nashville for several days now -- and the longer Riley is missing, the more anxious they get.

Riley was last seen on March 8, and there's been no sign of him since. Cops are encouraging anyone with helpful info to call 615-742-7463.