I recently learned with interest of Elbit Systems' Seagull unmanned surface vehicle (USV). It's designed to hunt both underwater mines and submarines, operating at sea in the same way that an unmanned aerial vehicle (UAV) operates in the air.
According to initial reports, a Seagull system consists of two vessels and a control station, which can be either ashore or on another ship. The USV's are about 40' long, can cruise at up to 30 knots, and can operate for four days at ranges of up to 62 miles from the control station. Each can be tailored to carry whatever systems a client needs, whether anti-mine or anti-submarine. Payloads can include, for example, the Katfish towed sonar platform from Canada (shown below), which allows high-resolution seabed mapping (essential to locate mines on the sea floor) among other functions.
A complete Seagull system (both vessels and the control station) is said to cost about $30 million, but in littoral waters can do the job of a naval vessel such as a minesweeper, corvette or frigate that would cost many times more to buy and much more to operate (including a crew at least several dozen strong). Here's an Elbit promotional video showing the system in operation in an anti-mine scenario.
This development isn't unique, of course - it's merely the latest to be announced in the field. However, such developments indicate how fast the anti-submarine and anti-mine-warfare fields are developing. Until relatively recently, such operations required large, dedicated warships, such as the US Oliver Hazard Perry class frigates or the newer (and seemingly much less capable) Littoral Combat Ship program, or Britain's Sandown class minehunters, or equivalents in many navies. Each ship would cost hundreds of millions of dollars, and require large crews.
In littoral waters at least, the cost of one such ship will buy up to ten Seagull systems (i.e. up to twenty USV's and ten control stations). The latter will be able to patrol a much wider area, much more frequently, at a vastly reduced running cost compared to conventional vessels; and if one is lost to enemy action, no highly trained crew members are endangered. Their higher-technology sensors (and, presumably, weapons such as lightweight missiles like the US Hellfire, stabilized light cannon and machine-gun platforms like Israel's Typhoon, and lightweight torpedoes like the European LCAW, none of which require a crew to operate them) will render minelaying or submarine operations in such waters much more hazardous, and may deny access to them altogether.
Even relatively poor countries that can't afford the expense of a 'proper' navy may be able to afford multiple Seagull systems or their equivalents, thereby protecting themselves against underwater intruders at a much more reasonable cost. This can (and probably will) affect submarine operations by the major powers, who will no longer be almost sure to have freedom of movement in parts of the world where no major anti-submarine warfare capability has existed before. This applies particularly in shallower waters, affecting activities such as recent intrusions into Sweden's territorial waters or the US Navy's famous Operation Ivy Bells in the Sea of Okhotsk during the Cold War. A series of Seagull-type units guarding such waters, backed up by a few more conventional warships, might have made such operations impossible.
I think we can expect to see anti-mine and anti-submarine operations become much cheaper, much more effective, and much more widespread in littoral waters. That, in turn, is likely to force a complete re-evaluation of many tasks currently allocated to or carried out by submarines. What will take on those roles in future, and what new equipment and tactics will evolve to deal with such new threats?