The Drowning Doom are a very peculiar faction. Outside of tier one, they are extremely weak alone, but are powerfull when grouped together due to debuffers. This means that you must get a fan advantage fast, as you do not have a unit that can single handidly take back geysers like Ironheade and Tainted Coil do. What you do late game depends on the early game; where most games are either won or lost.
DD's general stratagy early game consists of overpowering your opponent with gravediggers. Gravediggers have one of the single most effective double teams in the game, and will easily beat any other tier 1 units. It also only takes three double team hits to kill a leach! Keeping in mind that you must be able to adapt, this is a good way to start of a game on most maps (3 geyser ones like battered bluff)
1. Make 3 gravdigger squads
2. Plant rally flag at center geyser
3. Send 1st squad to right geyser as soon as it comes out
4. Send 2nd squad to left geyser as soon as it comes out
5. Leave 3rd squad to go to rally point, harrass enemy
6. Depending on what your opponent does either make more gravediggers or upgrade.
Generally, it is not a good idea to upgrade if you are behind in fans, especially because DD's tier 2 isn't very powefull. Ratguts are good if cloked, but can easily be picked off. Lightning Rods are weaker than headsplitters, and can generally be killed before killing a merch booth. Assuming that you won early game, upgrade straight to tier 3. At tier three you have your besr unit: the Dirigable. Make sure that you ALWAYS have one with your army. Generally my Army looks something like this:
3 upgraded brides
1-2 squads of upgraded gravediggers (these are great against razergirls and punishing partys)
some broods and maybe a frightwig or ratgut
Keep your army relativley grouped up and try to possess rockcrushers or any other powerfull unit. Good luck!
PS. Don't put the beacon directly on a geyser or your units will ignor attackers. Instead, put it right next to it.