Here I decided to not get the bright colored seawater in, instead, I was aiming for a more natural total.
When I'm travelling I often do without the brush cleaning spirit: white spirit. Then I'll only have my oil paints and no medium at all.
When you don't use any solvents, not for rinsing or other, and you only have the paints, you might need to use more than one brush, one for the brightest areas, one for the darkest. One per color-group perhaps.
A lot of wiping off the paints, because you can't really clean them while painting. It has benefits though: you will be using 10% of every color in every other color, so if you can make it work, you can reach color-harmony perhaps faster. Test it out!
After a day of painting here I just put my brushes in water (keeping them from drying) or I cleaned them in olive oil, washing them with soap after.
I teach about painting Seascapes in ► Seascape Beginner that's here and an online course more specific about Summer seascapes, figures and parasols is this one: ►Painting Summer Seascapes find this here.