Sure, but it's not like a cleric not bringing protection from energy is going to have their level 3 spell slots sprinting 10' a round at something that can crawl away prone at 15'. Not having the right niche spell doesn't prevent you from leveraging your spells at all, but an encounter that starts with 30 or 40' between you and enemies makes your ranger's entire flagship build defining feature that holds all of the build value for several of your levels do nothing for 3 or 4 rounds if you happened to think the boat adventure would have fights in the water when the enemy instead just jumps on to your boat.
For longstrider: See OP. Spending spell slots to do that all the time is not going to be good, and waiting until combat starts means you have to be an entire turn behind when fighting starts just to have your extremely slow octopus still need to spend actions to catch up with something merely walking away.
This would in fact be great, but in terms of mechanics, if you're stuck to RAW ala something like adventurer's league, or just how most DM's do things in my experience, making it "ride you" like you would a horse requires you to be large, since the beast of the sea is medium, and regardless of what you do to get around it, a DM can veto it by saying you don't have the proper anatomy to be ridden. The other option is to grapple and drag it, but then your speed is halved, and you might be bringing your archer into melee range just to do this janky thing.
I also see a lot of "just use the other ones", which is ironic considering the same argument could be leveled at criticisms of the PHB ranger, which has a lot of fixes in this book, by saying something like "if you want to play a nature warrior, just be a fighter with a leaf on your head".