The two completed their deals which are set to net the Millers around a million pounds earlier in the day.
"The main difference will obviously be playing in the Premier League, but it's a place I've always wanted to play and it will give me an opportunity to prove myself.
"It will be a big help to have Will here with me. When I moved from Mansfield to Northampton I didn't know anybody so at the start it was a bit lonely, but with Will signing as well it will make life a bit easier."
16 goal Hoskins was equally pleased and commented: "Watford's a massive club. I've heard great reports about the manager and hopefully he'll bring my game on.
"I have belief in myself and my family have belief in me, and hopefully I'll do a good job for the club.
"I'm a player who thinks you can score from anywhere, if there's an opportunity I'll try and get it in the net - anyhow, from anywhere. I'm an intelligent player, I'll work for the team and I'll try to bring others into play.
"I'm not just a hold-up striker, I try and make intelligent runs and being here hopefully they'll get even better."
The ex-Miller was also happy to have Willo joining him in the move South, adding: "It's definitely a bonus for me having Lee here. It's half the battle, on your own it can be a bit daunting starting at a new club. Coming in with somebody I'll be able to get with on with things a lot quicker.
"Ever since I've been playing with Rotherham it's been a relegation battle so I'm used to this sort of situation. I helped them out of it last time and let's hope we can do it at Watford this year."