I had heard that one should not go on a boat during the 9 Days because going on the water is dangerous and one should not go on a dangerous trip during the 9 Days. Is there any truth to any prohibition of riding on a boat during the 9 Days? If so, is there a difference in the prohibition if one goes on a cruise, a fishing trip, or a local ferry?
This is not explicit, but I think it is a logical extrapolation. R' Shternbach writes in Teshuvos V'Hanhagos 2:263
ולדעתי מותר להתרחץ בים, אבל לא לשוט עמוק ער קרוב לקומת איש שהימים האלו ר"ל הם מסוגלים לסכנה ח"ו... ובכל השנה לא חיישינן לסכנה רחוקה , אבל בימים אלו ראוי למנוע וכמבואר בש"ע
In my opinion, it is permitted to wash off in the sea, but not to wade deeper than the height of a person, because these days are days of danger. The entire year we are not concerned with far-off dangers, but in these days it is fit to refrain.
R' Shternbach prohibits wading into the water deeper than one can stand, because of the potential danger. Going on a small boat would, in my mind, be no less dangerous than going into water deeper than one could stand.
Shalmei Mo'ed quoting R' Shlomo Zalman Aurbach:
ומהאי טעמא לא היה ניחא לרבינו כלל הנסיעה במטוס בט׳ הימים שמר״ח אב עד תשעה באב, ואע״פ שבזמנינו נסיעות אלו מעשים הם בכל יום, מ״מ הואיל ומברכין על כך ברכת הגומל, אין לאדם להכניס את עצמו לספק סכנה בימים אלו
R' Shlomo Zalman extends the prohibition of danger to air travel. Even though his specific reasoning is because we say "hagomel" for air travel, he is comfortable extending the prohibition of dangerous activity to things which he acknowledges are normal, daily occurrences. In Halichos Shlomo (2:14), R' Shlomo Zalman also prohibited swimming, so it is clear that his threshold was not those things on which we say "hagomel."
The source for not doing dangerous things in the three weeks is in shulchan aruch orach chayim 551 siff 18. There are two specific things mentioned. Not to walk alone from the fourth hour untill the ninth hour because of 'ketev mriri', and not to hit the students. The mishna berurah adds from the elya rabba not to walk between the sunlight and shade. The piskei tshuvos bring from eshel avraham mibatchetch and aruch hashulchan that the danger of walking alone is only outside of the city in the desert where nobody goes. And he brings from kaf hachayim not to sit or sleep in direct sunlight. It seems the common denominator is that the danger is from heat related sources, not water. Also in biur halacha he says to be carefull from beginning of tamuz. Anything else that people don't do has no mekor apparently.
The Moadei Yeshurun in the laws of the 3 weeks halacha 41 writes that one may ride on a boat but not on a canoe that is unstable. It's intresting to note that the reason of not going on a canoe according to Rav Moshe Feinstein (footnote 164 in Moadei Yeshurun) is not because of danger but because of rechitzah,since it's common to fall in water when canoeing. In addition, if the boat ride is for pleasure that would also be frowned upon during the nine days according to Rav Moshe as cited in halacha 40.