Our company sells Kayaks and Kayak accessories and we would like the ability to allow customers to buy a kayak and receive 15% off everything in our accessories collection. Weather they buy 1 accessory or 20 accessories we would like them to get 15% off all of it automatically. Is this rule possible with automatic discounts?

Yes, that's possible, with 4 possible approaches.

Using a Basic Discount Rule

With % discount:

1) Create a collection-specific discount on your Kayak Accessories collection
2) Create a Basic discount rule using the discount code of #1 above.
3) Set the rule to "# of items >= 2" (or Cart amount >= $X) or (# of different items >= 2)
4) Add an additional restriction based either on the Product Type of your Kayaks, or on the Variant title of your Kayak. Let's take only the Product Type for example, if you have more than one product type, enter them all. Make sure that no Accessories product have the same product type than your kayak.

With a Buy X get Y discount

1) Create a Buy X Get Y discount. for "X", select your Kayak collection. For "Y", select the your Kayak Accessories collection and put a 15% discount.
Make the same #2, #3 and #4 steps as above.

Using an Advanced Discount Rule

(extra $49.99 one-time fee)

With a % discount

1) Create a collection-specific discount on your Kayak Accessories collection
2) Create an Advanced discount rule using the discount code of #1 above.
3) Set the rule to "# of items >= 1" (or Cart amount >= $X)
4) Select the products to trigger the application of the automatic discount (all the products of the Kayak collection. Eg. https://gyazo.com/28293944332e1c0376c33379841a917d)

With a Buy X get Y discount

1) Create a Buy X Get Y discount. for "X", select your Kayak collection. For "Y", select the your Kayak Accessories collection and put a 15% discount.
Make the same #2, #3 and #4 steps as above.