Classifying boat images
By looking at your code snippet, I realize you are training your CNN from scratch. 

Use Transfer Learning Instead. Training a new model (choice of model architecture i.e. how deep your model should be, hyperparameters etc.) is very difficult if not impossible with only 1500 images. You can achieve great results quickly by using  an already-trained model (aka Transfer Learning). If you are not quite familiar with the subject, read this article Transfer learning from pre-trained models, or this one First steps with Transfer Learning for custom image classification with Keras. There are codes included that helps to get started faster. One of the recent advances in Transfer Learning is efficientnet, you may want to jump using that one! But I would guess boats would be easy even with earlier models.