Using watermarked photos, in my opinion, is only ok when you show "the road". So for example part of your portfolio is step by step brief - research - sketch - concept - finish product. Then yes, by all means, until the point of final design use and show watermark materials (getty and istock watermark looks really professional).
Using many photos in "the road" stage (so until final design you change the watermarked photo) show that you are flexible, able to change and don't stick to one solution.
And as a designer don't be afraid to use your powers in your favour. So use photos with warm colors showing smiling, friendly people. The ones who will look at your designs will associate you with this "nice feeling".
In any other case buy the material, and don't be afraid in investing your money.
A) you will have some stock to use for your own use (freelance)
B) you will have nice photos to use in future mockup designs (many times I've had to put photo without watermark in Lorem ipsum stage)
C) it shows that you are willing to take one step more and showing initiative (you bought the photo and not the company you've work for)