Is it time to move from a 2 layer board to a 4 layer board
It doesn't seem that a 4-layer board is strictly called for in this case.   A 2-layer board would be sufficient.

There are no high speed signals on the board.  I2C is not a high enough speed to worry.
The board will work in a low EMI environment.  The level of EMI underwater is probably as low as it gets.

Other comments

Gridded layout (aka Manhattan routing) doesn't work as well for boards that have mostly SMT components, and many of them are quad-pack, and there's not a lot of spare real estate.

I would change the layout of the LTC3525 boost converter.  It should look more like fig.3 in the datasheet.  It's great that you have built and tested a batch of these devices.  If you further improve the layout of the boost converter, you should have less EMI.  That may lead to better readings from sensors.