Seems like these two things are the things that always break in bass boat engines. Why?

What can be done in terms of maintenance to reduce the likelihood of having a blown powerhead or lower unit?

They are the most stressed part of the motor...well those two things together basically make up the motor. I think power heads blow so often on outboards, especially on bass boats, is due to the amount of time the motor is ran at or near its maximum operating RPM. How often is a car's engine or even another two cycle engine (dirt bike, lawn mower)ran as hard as boat's out board? That said some outboards will last forever. The 1974 Evinrude I have on my boat has never been rebuilt and still has almost factory compression.

Lower units go bad for much the same reason, their parts are run at close to their operating maximum most of the time. Lower units are also subject to other abuses such as being struck by objects, or fishing line wrapping around the prop and destroying seals. Lower units are the lowest point of the boat when it is in the water most of the time and they are often hit buy hard objects at varying speeds. All it takes is one gear tooth to get out of alignment or broken to grenade a lower unit.

As for prevention, its just like anything else: frequent preventative maintenance and sensible running of the motor. Service your lower unit every year, and have your motor serviced as regularly as the manual suggests. Little things also help. Put fuel stabilizer in your tank if you know the boat may sit for a month, run ring free (or whatever Mercury calls it) every third tank of gas, check your lower unit seals and oil between services. Find a good mechanic that you trust and ask him what he might suggest.

Most importantly, pay attention to you motor, get in tune with it. Know what it sounds like when it is running good, shut it down if you sense something is wrong. I have owned numerous boats since I was sixteen years old and have never blown a powerhead, or had a lower unit fail, and all of my boats were older and well used.

My only other suggestion is buy an Evinrude :+

Lotta ins, lotta outs, lotta what have-yous

Lower units seem to fail when water mixes with the gear lube or its ran too hard and the gear lube is bad (too thin, no protection)

I would suggest replacing you lower unit oil on a regular schduel and replacing the washers behine the gear lube plug yearly to prevent water from getting into the lower unit.

Power head normally fail when you mix (on 2 strks) if off if you are not premixing oil injection tends to have problems especially on older motors when the temp is cold therefore the oil is thicker and does not mix as much oil with gas

other causes are over reving as john mentioned over triming is a popular cause of this

Hey let that Bass go I wanna catch her some day
http://www.calfishing.com/gallery/v/members/tmcustoms/

Here are the routine things I do to try and prevent things from going bad with the lower unit and power head.

Power Head: I rarely run WOT unless its tournament morning or I am late for weigh in. General fishing I try and vary the RPM's and basically take it easy. I over maintain my boat. I do full tune up to the boat about every 5-6 trips or so. It has gotten to the point I can, load it with grease, flush it with Merc. power Tuner change plugs and be done in about 15 minutes. I have a 225 EFI I will admit it fowls plugs pretty damn quick and that is not good on the power head. I can hear it when it is getting close to time to get it done. The only maintenance I do annually is Water impeller and Fuel Filter. Sometimes more often with the fuel filter if I feel I might have had poor grade gas. I have also heard that not letting the motor warm up before going WOT can cause powerheads to go. I pay very close attention to this. This could also be the reason it seems we hear more powerheads blowing during the winter and spring, rather then summer.

Lower unit: Leaking seal letting water in the gear case. Biggest problem from my understanding. I change my gear case oil every 5-6 trips. Watch for water, its not uncommon to see small amounts of water. You will know when it is to much. A few things that can make a seal leak. Normal wear, fishing line and not keeping the boat in the water on pad. I pull the prop off the boat almost right after or right before every trip to the delta or clear lake, looking for line or anything else that doesn't belong. Not as often on the big clear reservoirs.

Some might think the above is all overkill, the motor seems to run really well when overmaintained. bottom line take it easy on the motor and pay attention to the little things.