I google searched 8903.99.90 OTHER and come up with the following;
- - - - -Of an overall length not exceeding 9.2 m:
NMB 11 - - - - - -Outboard motorboats, metal, of a length not exceeding 5 m ...............
NMB 12 - - - - - -Outboard motorboats, metal, of a length exceeding 5 m .....................
NMB 13 - - - - - -Outboard motorboats, of reinforced plastics, of a length not exceeding
5 m .......................................................................................................
NMB 14 - - - - - -Outboard motorboats, of reinforced plastics, of a length exceeding
5 m............................................................................................
NMB15 - - - - - -Canoes.................................................................................................
NMB 16 - - - - - -Rowboats .............................................................................................
NMB 19 - - - - - -Other ....................................................................................................
NMB 20 00- - - - -Of an overall length exceeding 9.2 m.....................................................
Here is the original link if you want to go in and click and then CTRL-F the tariff code;
https://www.cbsa-asfc.gc.ca/trade-commerce/tariff-tarif/2013/01-99/ch89-2013-eng.pdf
Looks like it's 2013 so there maybe a newer one, however it seems to be to suggest that all outboard boats, weather FRP (Fiberglass), Metal (assume aluminum alloy), of all lengths (thanks to sub-class NMB 12, 14, and 20) are granted relief from the tariff.
These reliefs are listed in Schedule 4 of the Canada doc as linked above;
https://www.fin.gc.ca/access/tt-it/rcsa-rcmaa-eng.asp
Now what's interesting is the Canadian governments' wording in this April 19 2019 document. Here they say;
"
- For goods in Schedule 4, relief is granted for an indefinite period for other goods imported from the U.S. on or after July 1, 2018, subject to applicable conditions, as provided in the Remission Order.
"
So if I was someone who had imported between July 1, 2018 and now, I would be contacting my customs brokers to apply to get my 10% import tariff back retroactively.