Saints started well with Dusan Tadic's effort, before Capoue opened the scoring after nine minutes. Chances from Shane Long and Redmond followed but couldn't find an equaliser in the first period.
The home side got themselves back in it with Redmond's 58th minute equaliser, which was almost instantly canceled out after Maya Yoshida's horrific backpass. Watford had to hold out for the closing stages, after Ben Watson's dismissal, with opportunities from Tadic, Long and Redmond's disallowed goal, but both sides had to settle for an opening stalemate.
Against the run of play
Optimism was high around St Mary's Stadium ahead of kick-off, with two of league's new managers, in Claude Puel and Walter Mazzarri facing off against eachother.
It was Saints who started the brightest, having their first real opportunity of the game after two minutes.
Tadic was on a high, having signed a new four-deal on the South coast, and his effort could only be palmed well away by Heurelho Gomes.
His efforts were almost undone with a wicked defection from Miguel Britos, but Gomes did even better with the second save to claw it away.
Saints dominated the opening proceedings of play, but it was Watford who found themselves ahead with their first real attack.
It was a good move from the Hornets, with Nordin Amrabat finding Troy Deeney at the back post. His header back into the area was perfect for the onrushing Capoue, who slid in to strike his first competitive goal for the club.
Momentum lies with the Saints
Despite taking the lead, it was the home side that had the majority of the opportunities, as they looked to find the equaliser.
Tadic showed his excellence once again in the 33rd minute, slithering his way past three defenders before finding Long at the back post, but striker headed wide.
Redmond was one of the new signings brought in by Puel, and looked to make an instant impact as he received a clearance on the edge of the area, but his volley was well wide.
Banging down the door
Southampton came out for the second period as they had ended the first, and were eventually rewarded with their equaliser.
Redmond had been banging on the door with two initial efforts, but a poor clearance from a resulting corner was perfect for the winger. Gomes' failed punch fell in the air for the former Norwich man, and he showed great concentration to fire a low volley into the bottom corner.
All their efforts to get back into the game were almost immediately erased. A poor pass back from Yoshida gifted Deeney a chance, but luckily for the Japanese international, Fraser Forster was alert to clear his lines.
Hanging on by a thread
Saints were beginning to turn the screw, and the away side's job was made even harder, as they were reduced to ten men.
Southampton broke away from a Hornet's corner, with new signing Pierre-Emile Højbjerg sending Long on his way. The striker's pace was too much for Watson, who brought down the Irishman, and Roger East had no option but to send him on his way, a little over ten minutes after coming on.
The home side piled forward with the numerical advantage, Gomes was troubled again by Tadic from the subsequent free-kick, but his failure to catch once again remained unpunished.
It looked to be perfect for Long in the 78th minute, as Matt Targett did well to put the chance on a plate but one of the most prolific strikers in recent top-flight history, fluffed his lines and the hunt for a winner continued.
Redmond thought he had done it five minutes from time, Tadic and Steven Davis combined brilliantly. Davis played it through to Redmond who struck home, but the flag for offside denied him debut glory.