I've been reading boating books, many written in the US, for a few
weeks now. They talk a lot about boats like 30 foot sport fishermen,
but when I wander about the local marinas here (Helsinki, Finland) I
hardly see those boats at all. Some other types seem very rare as well,
such as bowriders, bass boats... not to mention pontoon boats.

We do have some variety too, but some examples on what kind of rough
(power boat) categories are common:
-smallish open boats, 4-5 meters (~15 feet), very abundant class, eg.
Busters http://www.buster.fi/?id=787. This brand makes aluminium boats
and is very popular, but many boats in this class are in fiberglass as
well, of course. Some boats are in ABS plastic. Side consoles, center
consoles, etc go under this class...
-hard top boats, looking usually something like this:
http://www.boat.no/hansvik/hansvik-18-hardtop-3.JPG.
-runabouts (many with covers to shield the boat/you from the elements)
-cruising boats (touring/travel boats?), like
http://www.ostamyy.com/veneet/kuvat/scan_511_468.jpg (a bit large
specimen here)
-sailboats could be a bit over 10%, maybe 15%, of all boats around here

A small minority of boats are "new" while most boats date back to 90's,
80's, and earlier decades...

I'm wondering what all kind of factors can cause these differences
between countries and locales. Can anybody comment on differences
between different states in the US or what kind of boats people use in
the European countries, and in the rest of the world... Japan etc?

It's obvious that the kind of climate and the kind of waters you
operate in has an effect. Runabouts, hardtops, cruisers and sailboats
offer some protection from the elements so that probably makes them
popular here.

Socio-economic factors play a part. The Finnish society feeds an
abundant middle class which results in lots of small boats. Shiny
express cruisers and yachts are rare, concentrating in areas like
downtown Helsinki. Contrast with the US which caters more to the upper
social classes, financially speaking. Different factors can influence
who takes up boating in the first place. A wet marina slip for a boat
with a 10 foot beam can be under 250 USD per season in Helsinki. I
gather in certain places in the US or around London for example that
figure is much higher!

I wonder how much the local culture or the mentality of the population
affects the choice of boat type. These factors may be hard to
distinguish from the ones above. In the US, large power boats may be
popular because the nation as a whole likes to burn oil in internal
combustion engines. But this is also economicly hardwired in low
taxation of gasoline. The absence of sports fishermen in Finland could
be explained by culture but also we don't have any oceans nor tuna to
fish in it...

I don't know why bowriders are rare around here. Perhaps the US boating
population places much more importance on water sports (not too popular
in Finland I think). This could be connected to the cold climate and
roughness of waters (minor sea) around here. Fishing I think is by far
the most popular activity to combine with boating in Finland.

I hope you found the speculation interesting,
Risto Varanka