I had an NGO chapter here in Central FL, working on engineering solutions for ship-strikes (and debris that causes entanglements). I had had a project (&, presented proposals at my work) to work w universities on why strikes occur and the best methodology to mitigate them.
Manatees' hearing thresholds are often outside those frequencies of boat motors...this means, they simply can't hear them coming. In addition, there is a phenomenon called "shadowing" whereby ships (large/small, etc) are essentially silent in front of their bows. Now, I won't go into the acoustics, but imagine if you are a whale in front of a large barge/cruise/container ship & all its sound is masked/attenuated...some ship-staff aren't even aware of this phenomenon...Yes, the sonar from the ship may detect the whale, but I've witnessed instances where (usually container ships & personal watercraft) ignore such information; when marine-animals' distributions are suspected/known, NOAA (at least in US waters), will send a signal/alert/warning to mariners. If the mariner causes either a "close-call" (w/in 500 ft of animal) or strike, NWS law enforcement will fine the perpertrator.
Also, whales sleep at the surface...very precarious...I had proposed using one of our UAVs to monitor, either infrared or SAR, to detect & create a potential distribution of Northern Right Whales...due to gov't budgets, I was unable to continue w the project. Please know, however, some universities and the US Navy are attempting similar projects to better protect marine fauna.
Please know that most of the Northern Right Whale mortalities are due to ship strikes to their mandibles (and, entanglements). Most of the perpertrators were foreign-flagged vessels (they either pay the fines or ignore...) and surprisingly, personal water craft. I have heard (don't have the statistics to support this hearsay) that blue whales near the Sri-Lankan coast also have high mortality due to ship strikes...it's a tough problem to solve, internationally; unfortunately.
2003: HAL Ryndam (Rio - Valparaiso (Falklands))
2005: HAL Noordam (Europe, Norway)
2006: NCL Pride of America (Hawaii)
2008: HAL Rotterdam (Brazil, Africa, Europe (Canaries))
2010: NCL Jade (Mediterranean (Turkey, Egypt))
2010: Azamara Quest (Dubai, India, Thailand, Singapore)
2011: Princess Diamond (Alaska, Hokkaido, Russia, S Korea, China)
2012: Swan Hellenic Minerva (Sri Lanka, India, Burma, Malaysia, Singapore)
2015: Sun Princess (Papua New Guinea)
2015: HAL Volendam (Cambodia, Vietnam, Brunei)
2016: NCL Sun (Buenos Aires to Santiago)
2016: Cunard (Cape Town, Namibia, Atlantic Islands (St Helena, Ascension, Cape Verde, Canaries))
2016: HAL Zuiderdam (Baltic, Norway, Iceland, Greenland)
2017: NCL Star (Middle East, Suez, Israel)