While the special effects are indeed impressive for its time (underwater shots!) and the scope of the film rather massive, it's a bit slow. Also, some aspects of the film have not aged well and are a bit problematic with modern eyes.
In addition, it feels like the film wants to make use of all the underwater footage it shot, so those scenes are unnecessarily long. Parts of the plot also feel strange to me, especially since I have not read the source material.
All in all, I'm glad that I have watched the film, and I am willing to give it another go if it receives a proper restoration, but it's not a must-watch film (unless you are interested in the technical aspects of filming underwater or so).