First, I'm assuming the units you posted above are what you brought (I'd normally say list but I have no idea how your squads are actually kitted out, just what you have available.)
Which means a watch captain with no upgrades, something that was strongly advised against, a pair of terminators with no upgrades (so why bring them?) Those are three easy targets for first blood, kill points, and warlord kill in most games.
His deployment saw almost 3 units arriving within 3" of mine and alot more around 6". They even got to shoot, that's right shoot in the deployment phase of all dam things
Thats called cult ambush, a rule that can cause a lot of damage very quickly. I'm assuming your opponent was running the cult insurrection detachment and had a subterranean uprising formation in order to maximize his chances of getting the best results. (Basically that formation lets the units it contains roll 2 dice for cult ambush rather than 1.)
fighting 2 or 3 units a piece which resulted in a majority of my army wiped in his first turn.
The strength of the genestealer cult army is the sheer numbers they can bring to bear. A unit of 5 acolyte hybrids is the price of a deathwatch terminator and comes with 15-20 S4 attacks with rending. Metamorph squads can take whips, making them I7, again with 3-4 rending attacks.
Against genestealer cults, range, ignoring cover, and focus fire are the best tactics.
opened fire onto one of his walker dudes and got myself 2 wounds on him and one wound on another large walker dude.
Initially I thought a crisis suit and then a riptide, but then your next bit..
His shooting phase saw 2 markerlight squads negate all cover and a Large blast marker wipe the whole squad instantly. Now the other walker dude which is apparently a forgeworld one jumped like 30" in his movement phase and wiped all but my vet sarge with a Strength 7? (i think) AP fucking 2 flamer, ignore cover and some other such bs rule completely wiping everything.
I don't know what was using a large blast weapon, especially with ap3, but the forgeworld unit was the Y'vahra battlesuit. Its a forgeworld variant of the riptide battlesuit found in the regular codex, that has the ability to move like a flying monstrous creature (though it cannot do that for two consecutive turns), can jump into ongoing reserve, and has a torrent flamer that can either be ap3 or ap2 with gets hot (both are S6.) Its also 230 points and a fast attack choice in a tau empire army, so I'm not sure if your opponent was able to use it in a farsight enclave army.
scored 3 wounds against him taking him down to 2 hull points.
Bringing it down to 2 wounds, as a battlesuit its not considered a vehicle. I will say I am kind of surprised you were able to do that, since its T6 and has a save the equivalent of your terminators.
So yeah his squad teleport right in front of the squad which killed the dread knight and wiped it out in that assault phase.
Uh, correct me if I'm wrong but gate of infinity is treated like deep striking, so how did that squad charge in the same turn?
So all in all the day was a complete slaughter for the old deathwatch and make me reconsider my life choices,
To be fair, and nothing against you, but you did go into a tourny with a non optimized list that you did not appear to know that well. Thats the kind of place where you really learn how much you know, and how good you are at adapting to situations.