One of my favorite ways of keeping my acrylic paintings clean from line drawings as I work is to use tracing and transfer paper. You can choose to freehand or trace your subject onto the tracing paper, that part is up to you.
With acrylic paintings, because that paint dries so fast, I will paint my entire background before I paint the subjects. If you pre-draw out the subjects and try to paint around them, the paint will start to dry as you carefully avoid painting over the subject and you can end up with brush marks where you didn’t want them.
As you can see from the photo above, I’ve painted my background completely before drawing in the whales and coral. This allows for a perfectly seamless background!
I can then tape my drawing onto the canvas and use transfer paper to transfer that image cleanly onto your painting. I prefer the white transfer paper from Loew Cornell. You will use a ballpoint pen (ideally one with no ink in case you puncture the tracing paper as you work) or a stylus.
I use tracing paper vs another type of paper for two reasons. First, it is thin enough that it works very very well with the transfer paper. Second is that you can see your background through it so you can more easily see where to position your subjects.