Heat gun is definitely worth it. As an alternative, if you are careful and doing the work in the open and not an enclosed space, you can also use a propane torch.
Also worth spending the little bit of extra money and get proper marine grade adhesive lined shrink tubing. I've had good luck with Ancor and Vertex products. The tubing is thicker, the adhesive seals out moisture and the shrink ratio is 3-to-1 vs 2-to-1.