Yes, the Webasto water heater wiring harness 1320454A 9001080A is suitable for the Webato Thermo Top C on a Narrowboat with the heatmiser fitted.
You may also require the timer adapter lead - Webasto Thermo Top C to TM1-N Timer Harness | 4110553A | Butler Technik
This will allow you to make the connection from the wiring harness to the heatmiser.
You will need to also make sure that you have appropriately sized cables for your battery connections. It is possible to extend or shorten the power cables for the heater, if you extend the cables you may need to use thicker cable.
Link to Heatmiser for info - Webasto Marine Timer Programmer Thermo Top 12v | 4118180A | 4110551A | Butler Technik