Getting Portable hottub, do I need to reinforce the deck?

Water is approximately 8# per gallon. Take weight of tub and add the weight of water when filled. That's how much weight your deck needs to hold.

Then look at joist system: joist size and length.

Figure out how much weight can be supported per sq ft.

A tub of water is much weight.

have a county zone and or safety inspector come out and inspect the deck. It's highly unlikely that it'll be able to withstand that much weight unless the deck is directly on the ground.

"Looks to be in good shape" isn't good enough if it breaks with people either in it or under it.

Like electiricty, this isn't something you want to "kinda hope you go right".