Velsen / Ymuiden is a difficult area to make good pictures, because of the many wind turbines, factory chimneys and Tata Steelworks in nearly every background, standing of the south side of the canal.
Most visited for shipspotting is Sluisplein, near the DFDS ferry terminal, free parking.
Then there is Pontplein, were the ferry Velsen-Noord/Zuid is situated (ferry is not free), free parking.
Difficult site is Vlissingenstraat near the Felison Cruise Terminal, shipping can be seen but you have to climb a rocky breakwater first.
Rather good site is Velserdijk in Velsen-Zuid, you can sit alongside of the canal and see the ships at distance in the locks, arrivals from Amsterdam are difficult to see (just pop up behind trees), free parking.
Best site is the southern breakwater, but you have to pay for parking and the parking area is closer to the beach than the breakwater.
Note that to road across the three sea locks is closed towards Beverwijk and Velsen-Noord because of construction of a new fourth lock. Therefore the former good viewing points between the locks cannot be reached anymore.
Take a look first with Google Maps / Streetview for position of the streets, parkings zones, etc.
Note that to compared to Rotterdam and Antwerp areas the shipping is much less: one vessel every hour is a good day.