Having one or two spies can be a great asset on hydro. It’s difficult enough to even reach the objective when surrounded by enemy troops, but it’s nearly impossible to evade the troops and the lethal sentry gun spray. A good spy should be able to kill off at least one sentry gun each life and it takes much longer to rebuild a sentry gun to level 3 than for a player to respawn, so these kinds of buildings rather than troops should be spy priorities. With the sentry guns down your team should be much more able to gain a foothold in the enemy territory. Just make sure you keep them up to date on the sentry gun status.
Somewhat contrary to my previous point, spies should also make snipers a priority, since they can take out any class in one shot and often have very comfortable emplacements to defend from. Often the only classes that can displace them are spies and other snipers, so make sure you have at least some of these classes to counter enemy snipers.
As for footholds, they are an important step towards winning the stage. If you can only get into enemy territory with a slim chance of survival, there is only a slim chance you will be able to do any significant capturing. You need your team to push forward as a team so that together you can counter any kind of resistance more effectively than a single person could. Forward teleporters are great for this purpose.
Medics, medics, medics. The slog of these stages is a result of both teams’ control points being right outside their spawn, so the defence is quite continuous. It’s a game of inches, and every player an attacking team loses will take longer to return to the front than a defender, so it’s critical to keep as much of your team alive for as long as possible.
If you are having trouble holding off enemy attacks, remember the effectiveness of sticky bombs for plugging up entrances, essentially blocking off entire areas. If your team can clear an area out, demomen can move in, lay down a defensive sticky bomb perimeter and that area comes under your team’s control. Doing this repeatedly allows your team to progress forward. Maintaining control of certain mid map areas is often the most important objective – it should certainly come before capturing the enemy point chronologically. See my article on map control for more on why I say this.