I'm not sure what the regulations are in Oz, but you are describing a fairly low-powered bike. In the U.S. we have access to bikes up to 750 W and beyond. Also, we have access to eBikes that are not only pedal assist, but also have an independent throttle to allow you to start easily from a dead stop. I know they are not allowed in Europe. I love the idea of a belt drive, but that eliminates the option of a derailleur (more gears).
One reason NOT to go with high-powered bikes that haven't been designed extremely well is that the power can break the other components. A very good cyclist can put out 250w for 20min. The gears, cranks, etc. can handle that. But if you just use the same mechanisms for 750W or 1000W, at best you get much faster wear and at worst they just break - lots of evidence in the online bike forums.
However, while the law here in Oz currently says 250W maximum on-road, I do think that should be relaxed to 350W and possibly 500W if on a well-designed bike - or trike! Shouldn't forget the recumbents of course: I have a Greenspeed trike I'm going to electrify that should be great
.
Now the good manufacturers have done the work to make it work - tube size, connections, etc. But not all have, and people are still buying DIY 750 and 1000's to put on bikes that just won't take the power.
Now, there's also the cost in weight of bigger motors - they are heavier and while it doesn't make as much difference if they're in the bottom bracket as mid-drives, it does if they are hub motors in smaller (20"/24") wheels. As always, while the motor is working the extra weight is fine (except when lifting it on/off the boat!), but when it inevitably stops at some point, riding that bike will be much harder (if geared motor) or absolutely horrible (if not).
IGH - the Rohloff has more gears than you can get with a rear derailleur, and add in a Schlumpf forward and it's more than is possible with *any* derailleur (I believe). Look at the Pinion as well, or at the Alfines for fewer gears and much cheaper. All are sealed so no problem in salt water environment, as against the chain and derailleur which won't last (actual length of time dependent on lots of things though). On the other hand, a derailleur is much, much, much cheaper
.
I've never needed the throttle - the IGH can be shifted when you're still at lights or waiting, so starting in the right gear is not a problem. And power off the line when you can bump to maximum assist and the right gear has never been a problem!