Ich habe nicht verstanden, was Du mit "Navi eingestellt auf Grad xxx" meinst.
Wie gross ist die Abweichung denn? Besser als auf ein paar Meter, so zwischen 5 un 10 m, sind normale Navis nicht.

I have had GPS traces displaced for some hundred meters occasionally with a my Holux M241, which generally works well.
I've got the impression that sometimes the first fix after switching on goes wrong. Switching on the unit in a free space far away from buildings might help.

GPS does knew nothing about daylight saving time, that's not an isssue.

JOSM is working fine with Yahoo WMS for me.

I remember having changed something there, because it did not work any longer.
Maybe you should remove the Yahoo entry and put it in back from the supplied standard.
The entry for Yahoo reads:
Yahoo Sat html:http://josm.openstreetmap.de/wmsplugin/YahooDirect.html?

My second guess was a problem with webkit, but then I reread your post and saw that you're using a Mac. Sorry, I do not have a Mac.

There is no distance measuring tool on the standard OpenStreetMap site.
What you can do:
Start Potlacht. If you select a point, pressing "L" will show the coordinates. You'll have to compute distance from the coordinates.
Use JOSM. There is a measuring plugin in to measure distances.

Für einen echten See oder Teich natural=water, bitte die Kontur im Uhrzeigersinn anlegen, es gibt Renderer die sonst Probleme machen.
Im Falle künstlicher Wasserbauten käme auch landuse=basin oder landuse=reservoir in Frage.

The park is part of a relation, which does not seem right.

The relation is about buildings several miles away. The mysterious "DX group" comes from these buildings.

I would suggest to delete the relation from the park.

Take a look at osm.wiki/Harbour
As far as I understand, there is no specific tag yet.
man_made=pier, floating=yes is suggested for general case of rafts.

Falls Du JOSM benutzt:
Da gibt es die Funktion "rechtwinklig machen" (Q).

Wo sind gerade dort Lücken?

Verona ist ziemlich gut erfasst!