Your missing the point.....penny wise and a dollar foolish.
If all you want is depth, buy a dash mount meter for $90. If you want a useless fish finder to go with the depth, you can buy one of those for $100. Why spend $150 and risk damaging an expensive IPAD for something that could be done with unit designed for the environment for a lot less?
What's an IPAD going to set you back when it goes overboard or takes a dump from the environment?
You have a very good point and in this case where the transducer and software cost more than an entire standalone system the math doesn't work out. That said it is a interesting trend and I believe it is likely to be the future of marine electronics.
High end marine electronics are already interfaced together where components are less of standalone systems and more of add-on modules. You buy a high end plotter and connect a bunch of components to it to provide sounding, autohelm control, radar, infared vision, anemometer, etc. and they all talk via NMEA protocol. Sailing yacht electronics have been this way for years where you have multiple displays around the cockpit, base of the mast and steering pedestal and you change what data you want each to display.
The possibilities of tablet based systems utilizing bluetooth or wifi interface with any sensor device that pops up on the market would provide incredible flexibility and cost savings. This really the same idea behind the NMEA standard but it has already evolved through multiple versions and will continue to do so. As NMEA continues to evolve it will leave older systems obsolete and unable to expand unless you buy similar vintage equipment or everthing is backwards compatible, basically I don't think it is as universal as it was conceived.
Using wireless communication languages which are far more popular would drive down software development costs and probably bring more vendors into the market place. If you are using multiple standalone systems you are paying for the costs of the display, processor, and interface for each system. The display, computing power and interface capabilities of most tablets and smartphones are greater than most marine electonics. Marine instrumentation companies are not solely focused on computing power and interface, huge electronics conglomerates have far more expertise and buying power for the systems components that are not unique to marine systems. I expect we will find raymarine, garmin and lowrance turning out products which gather information and communicate it to tablets loaded with their apps. It may be more profitable for them in the long run. Garmin is already selling navigation applications to run on apple and android devices. I wouldn't be surprised at all if motor manufacturers would participate, they are trying to be and be perceived to be technologically advanced. You could easily have all information sent to a tablet on the dash eliminating the expense and effort of installing guages and cutting up your helm's dash.
Installation is another area where tablet based systems can shine. Far fewer cables to run or snake through towers, t-tops and radar arches. You just run power to the new sensor or a terminal block nearby, mount the unit and you are done. Waterproof cases are available for many tablets and some even have inductive charging capabilities so you don't even have to plug it in to charge it.
All of these things said there are certainly drawbacks and limitations to tablet and smartphone based system. You have a single point of failure for every system so you lose everything at once. The displays of most tablets and smartphones are lousy with glare and some are even polarized so they cannot be viewed with polarized glasses at certain orientations (my nexus 7 does this). Some times you just want buttons over a touch screen, when it is rough as hell and you are trying to make an adjustment a real button is easier to select than a point on a screen. Rain and spray can confuse some touch screens. I expect these challenges to be overcome or something most customers are willing to deal with.
Overall I don't see this specific sounder as being a game changer but a sign of major changes to come.
