NYPD will have both they will soon launch SRG and they will keep ESU too.
There's also really no shortage of hydrents in NYC. Every block has a few of them and traffic are quick to tow cars parked within 15 feet of one. As was already mentioned though, DCU's water cannons just sit in lots and wouldn't be filled in the field regardless. The inclusion of the LRAD would make the disused water cannons needless anyway.
I'm still inclined to guess it's a fuel tanker given how spiffy the fuel limit on helicopters was in the US Army mod and the fact that the NYPD does indeed employ a whole lot of these trucks for that purpose. The only other possible purpose that comes to mind would be using it to decon a hazmat area I suppose, but I don't know that these trucks have ever been used for something like that.
Side note on the SRGs: These are still most likely going to be ESU officers. It's likely that the SRG will end up being something similar to the NYPD's existing model for their Hercules teams which consist of mostly CTU and ESU officers. Considering that ESU already only consists of a few hundred cops it seems unlikely that the Commissioner's plan of having 300 officers assigend to SRG will mean an entirely new force of officers whom would be roghly the equivilent of ESU anyway.
As I understand it, NYPD SRG is being created out of the old NYPD task forces. Task forces were comprised of patrol officers and each patrol division (i.e. Manhattan North, Manhattan South, Staten Island) had a task force. Task forces were assigned to a) provide additional patrols in high crime areas, b) provide a rapid rush of manpower to major incidents (i.e. plane crashes, natural disasters, etc.), and c) handle disorder control (riot police). Last I heard, NYPD was looking to dissolve these task forces, which would leave them without a disorder control unit(s). Second, after the Mumbai terror attacks, members of the Organized Crime Control Bureau (OCCB) were trained in limited tactical measures in case of large scale terror attacks where ESU would need assistance. Likely, some of the task force members were reassigned to this new Strategic Response Group, and then given some tactical training, as this new SRG will be responsible for disorder control in addition to counter terror operations. However it is highly unlikely that this SRG will have anywhere near the same tactical capabilities as ESU. ESU has always been NYPDs tactical unit, for whatever the operation, and they pride themselves on being the best of the best. As their saying goes "When a citizen needs help, they call the police. When the police need help, they call ESU."
I'm not actually from New York City and I don't spend a lot of time there, so I won't get this 100% right. Only been once and that was back in 2005 and that was for 2 days.
The NYPD Counter Terrorism Unit (Or CTU) focuses on combating potential terror attacks in the city of New York and the NYPD coverage grounds.
It was established back in 2002 after 9/11 so that the NYPD could monitor terror activity within New York and Worldwide without relying on Federal Agencies, i.e. the FBI or DoHS
I believe that it's basically a unit that monitors worldwide terror activity and raises the readiness and awareness of NYPD officers based on what is happening.
For example, if there was a series of terrorist attacks in another country, let's say Spain (Not that there would be one specifically in Spain) for this example, then the CTU will acknowledge it and take necessary action, such as increase amount of units patrolling, have special units on standby, evacuate certain buildings, etc.
They also do have an armored division, so they are similar to ESU (I believe). I'm thinking the SRG will be similar to this division.
Actual New Yorkers like TheHockeydude might be able to tell you more if you are still confused. I apologize if what i'm saying is completely wrong, this is just based on what I read in about 20 minutes. 
CTU does more than acknowledge terrorist attacks across the world, they have officers stationed around the world who respond to these terror attacks. These officers then in turn interact directly with local law enforcement at the scene of the terrorist attacks and report back to the NYPD brass and analysts in New York in real time, who then use the information to coordinate the NYPD response. Therefore the NYPD is responding to possible terror threats in real time compared to other cities who may have to wait till the next day or two to get some version of a report from some other agency which may or may not address that city's concerns.
Finally, just to add, while NYC does have a ton of law enforcement agencies, who all call themselves police, most are not full police and or do not really deal with true police emergencies. Most of these agencies have peace officers, which is a lower standard than police officer, and peace officers are only allowed to handle crimes/incidents specifically under their jurisdiction. I.e. department of health and hospitals police don't carry fire arms and really have no power outside of property owned by the department of health and hospitals. Or the sheriffs office, who is under control of the department of finance and really only handles city tax enforcement and civil law. So, the only real full service police department in NYC, under control of NYC is the NYPD.
. Oh also TheHockeydude39 I was the one who said you could put any liquid in a tanker but you must of read wrong what I said was that if they didn't want to put fuel in the tanker then they could put whatever they want. It's their mod and it doesn't have to be realistic.
