The wire sizes (AWG8 vs. AWG10 or 12) aren't all that different. One approach that might work well for you, providing a strong, solderless, water tight connection, is based on the use of good adhesive heat-shrink butt connectors plus additional adhesive heat-shrink tubing.
1. Use high quality yellow butt connectors, like the Ancor ones sold at West Marine;
2. With your wire strippers, carefully pare down the AWG8 to AWG10 size (which the yellow butt connectors are designed for);
3. Slip some appropriate sized adhesive heat shrink tubing over one wire;
4. Crimp on the butt connector using a proper crimper for heat-shrink type connectors (which don't damage the heat shrink);
5. Shrink the connector; and
6. Slip the additional heat shrink tubing over the entire connector and wire ends, and heat shrink.
This will provide a strong water tight connection.
The use of solder probably isn't a good idea as solder will likely increase the propensity of the wire to break due to wire movement. With very few exceptions, electrical connections on a boat should not be soldered. Good crimps are way better.