I've been on lots of boats with underwater exhaust. They can work, but not a big fan. Unless designed right, backpressure can be pretty high or even over spec. I prefer a good muffler and tailpipe above waterline. Nice to be able to see, at a quick glance, that you have good water flow and a clean burn.
The IPS really needs a relief line. Constantly blowing bubbles under the boat is annoying.